What follows is a guide to making a series of do-it-yourself (DIY) fertilizers for aquariums that are the preferred fertilizers for the author. The author prefers ammonium based fertilizers that put the iron and phosphorus into the plants in the form of root tabs. It is followed by some data on the chemicals for fertilizing aquariums. It is VERY long and boring and only meant for real aquarium nerds like the author.
The DIY Fertilizer Regimen I Recommend
I’m not exactly happy with the composition of ANY of the commercially available aquarium fertilizers. They do not add the nutrients where they should be added. They all add the nitrogen as nitrate. And they are all very expensive.
So I prefer using a homemade fertilizer that has the following characteristics:
- Adds phosphate and iron to the substrate, not the water column
- Uses ammonium rather than nitrate, depending on pH
- Is MUCH cheaper than the bottled fertilizers you buy on the internet
Algae do NOT have Roots
This addition of iron and phosphate fertilizer to only the substrate of an aquarium is based on a very simple truth: namely “Algae do NOT have roots!”. Rooted plants and algae are both plants. They both utilize fertilizers, carbon dioxide, and light to produce their growth. So rooted plants and algae are always in competition in a planted aquarium.
The obvious way to give rooted plants a competitive edge in this battle is to simply add the fertilizer to the substrate. By adding fertilizer only to the substrate ONLY rooted plants can access the fertilizer and thus ONLY the rooted plants can flourish.
Most (but not all) rooted “vascular” plants have been shown by testing to absorb phosphorus and iron from their roots much better than from their leaves. Most (but not all) vascular rooted plants also absorb potassium and nitrogen much better through their leaves.
Many add a “complete” soluble fertilizer to the water of a planted aquarium. This is a fertilizer that contains nitrogen, phosphorus, and potassium. While this can be done very successfully, especially with epiphytic plants, one loses the potential of controlling algae by giving the rooted plants a necessary nutrient, phosphate, through the roots.
While potassium and nitrogen are so soluble that it makes little difference where they are added, phosphate typically has limited solubility in a fine substrate. One can SLOW DOWN but not STOP algae growth by adding phosphate only to the root zone of rooted plants.
Note this addition of phosphorus to the roots ONLY works with a fine substrate under 2 millimeter in size. A gravel substrate or an undergravel filter makes roots tabs useless. One might as well just add all the fertilizers to the water column. For fertilizer formulas which work with gravel substrates go to these articles:
15.5.5. DIY Epiphytic Fertilizer
Homemade Fertilizer for Adding to the Water Column
I recommend a homemade water solution of nitrogen/potassium fertilizer added to the water column of most planted tanks.
A weekly cumulative added dose of nutrients for a high-tech aquarium with a moderate amount of lighting, CO2, and plants, according to this Homemade Fertilizer program, would be around:
- N nitrogen level of – 3.4 ppm (This is nitrate or NO3 of 15 ppm)
- K2O – Potassium – 12 ppm
Or an “NPK” of 3.4-0-12.
A nitrogen and potassium water-based fertilizer for the water column makes up a water solution made with three chemicals obtained over the internet. Take one liter of hot distilled water and add:
- 90 grams potassium sulfate (0-0-55)
The potassium sulfate is very slow to dissolve. It may take a few hours and require reheating several times. Allow the solution to cool to room temperature then add:
- 76 grams of ammonium carbonate (29-0-0),
- 30 grams potassium bicarbonate (0-0-47)
- 5 grams potassium sorbate preservative
This creates roughly 1.2 liters of liquid fertilizer with a composition of roughly 3.4-0-12 NPK. Note ammonium carbonate can readily decompose in hot water so do not add it to hot water unless you like the smell of ammonia.
This water-based fertilizer is designed to be added every day first thing in the morning. The dosages are shown in the charts below in this article. Note electronic dosing pumps are VERY useful to add this fertilizer. And store all fertilizer solutions in the refrigerator to prevent mold from growing in them.
Ammonium is very rapidly consumed by stem plants, so it typically will be gone in one to two hours from any planted aquarium. And ammonia is vastly overrated as a fish toxin. So I use ammonium in planted aquariums.
Note that the timing of the dosing is important with fish in the tank. Add the fertilizer first thing in the morning, when the lights have been out for at least four hours. Note that ammonium is rapidly absorbed by stem plants even in the dark. And pH can rise rapidly in a good aquarium. So it is beneficial to have the lights come on one hour AFTER the fertilizer is added. But most hobbyists like to add both food and fertilizer to the tank first thing in the morning just after turning on the lights. This will be fine.
Do not add ammonium fertilizers after the lights have been on for any length of time. The pH of a planted tank can rise very high in a very short period and ammonium is poisonous at high pH. Note that timed dosing pumps are VERY useful for fertilizing any planted aquarium.
This fertilizer formulation is flexible. If one see new leaves with yellowing and the nitrate level of the water is below 20, add potassium nitrate, ammonium carbonate or urea. If one has holes in the leaves go ahead and add some potassium sulfate. If older leaves are turning yellow, add phosphate. Do not be afraid to experiment. Different plants respond differently to fertilizers and water chemistry plays a big factor.
Rigorous Fertilization Programs
Some hobbyists like to wing it and do their own thing when it comes to fertilization. Others want a rigid program that tells them exactly what to do. Either course of action is fine and can be made to work because Mother Nature is very flexible.
A “Complete” Fertilization Program for Low Tech Planted Aquariums
If the tank is low-tech without carbon dioxide injection add one homemade iron and two homemade phosphorus fertilizer tab under every rooted plant every five months (the recipe for these is below in this article). Then very frequently add the water-based fertilizer solution from above in this article.
Here is a useful chart for low-tech planted tanks for how much water-based fertilizer solution to add to the water column every day:
Again, take things slow and ramp up. Plants normally don’t absorb fertilizers very well for the first few months. Low-tech planted aquariums require patience. Note that the more frequently one adds this soluble nitrogen and potassium fertilizer the better the results. I used a dosing pump that added a dose one hour before the lights came on every single day.
Some useful conversions are:
- 1 tablespoon = 3 teaspoons =15 milliliters
- 1 teaspoon = 5 milliliters
- 1 milliliter = twenty drops
A “Complete” Fertilization Program for High Tech Planted Aquariums
For a rigid program for a high-tech CO2 injected high light planted aquarium put one DIY iron tablet and two DIY phosphorus tablets per rooted plant deep into the substrate once every month (the recipe for these tabs is down below in this article). Then add the water-based fertilizer solution from above in this article once daily first thing in the morning:
Here is a useful chart as to the amount of water-based fertilizer solution to add once a day to a high-tech planted aquarium:
If one has fish in the aquarium, add the water-based fertilizer solution after at least four hours of dark in the aquarium. This avoids any possibility of ammonium toxicity. Dosing pumps are very useful for making these additions. If you use a dosing pump add the fertilizer after at least four hours of dark in the aquarium but one hour before the lights come on in the aquarium. This maximizes absorption by the plants. Note that most hand sanitizer bottles dispense one milliliter of solution per push.
Again, the amount of fertilizer, carbon dioxide, plant mass, and light in a high-tech tank need to be “balanced” based on plant mass. If one has just started a planted aquarium with a few clumps of stem plants, use 10% of the fertilizer amount, 10 ppm CO2 and 30% lights on for 6 hours. If one has a very well-planted tank with a large mass of well-established stem plants, double the fertilizer amount, keep the CO2 at 30 ppm, and keep high-intensity grow lights at 100% on for 16 hours.
Note that the more frequently one adds this soluble nitrogen and potassium fertilizer the better the results. I used a dosing pump that added a dose of water-based fertilizer solution one hour before the lights came on every single day.
Some useful conversions are:
- 1 tablespoon = 3 teaspoons =15 milliliters
- 1 teaspoon = 5 milliliters = 100 drops
- 1 milliliter = 20 drops
Homemade Root Tabs
Plant tabs are small capsules that are inserted into the substrate to deliver the fertilizer rooted plants need directly to the roots. This will NOT stop algae growth in the aquarium. Algae can grow at a VERY LOW concentration of nutrients. Green spot algae and diatoms can grow at fifty parts per billion of nutrients. That is 0.050 parts per million. But the growth will be slow and plants growing at a decent rate can control this algae with chemicals (negative allelopathy).
One can easily make phosphorus plant root tabs using the following formula:
- 75% Plaster of Paris (calcium sulfate or gypsum) binder
- 15% Epsom Salts (magnesium sulfate)
- 10% Triple Phosphate (other names are “superphosphate”, superphosphates, and calcium dihydrogen phosphate), available from the internet. While the calcium hydrogen phosphates are preferred here, this can be any phosphate salt, including dipotassium phosphate, monoammonium phosphate, and diammonium phosphate. Note avoid bone meal (apatite) as it doesn’t decompose very well in the aquarium. Note I found all these phosphate chemicals difficult to find in some countries.
Note I don’t pay a lot of attention to the exact mix. Very roughly two tablespoons of Plaster of Paris, one teaspoon of Epsom salts, and two-thirds teaspoon of phosphate are just fine. This isn’t rocket science. Also, note that the Plaster of Paris (calcium sulfate or gypsum) is not a fertilizer. It is only a binder that slowly dissolves and releases the phosphate.
One can easily make iron plant root tabs using the following formula:
- 80% Plaster of Paris (calcium sulfate or gypsum) binder
- 15% Epsom Salts (magnesium sulfate)
- 3% ground pyrite (iron sulfide, internet purchase) or finely shredded steel wool. This addition is optional.
- 2% ferrous sulfate powder or iron chelate (garden store or internet). Any type of ferrous sulfate or iron chelate is fine. Ferrous sulfate is also known as iron sulfate, green copperas, and green vitriol (caution, blue copperas and blue vitriol are copper compounds). And any iron chelate is fine.
Note I don’t pay a lot of attention to the exact mix. Very roughly two tablespoons of Plaster of Paris, one teaspoon of Epsom salts, a one-fourth teaspoon of pyrite or steel wool, and a one-sixth teaspoon of ferrous sulfate or chelated iron are just fine. This isn’t rocket science. Also, note that the Plaster of Paris (calcium sulfate or gypsum) is not a fertilizer. It is only a binder that slowly dissolves and releases the iron.
The reason to make up two separate tabs is that phosphorus and iron tend to cancel each other out in a fertilizer. In a well-oxygenated environment, especially at high pH, the iron phosphates which form are very insoluble.
Note these tabs will not work in gravel. Large diameter substrates and under-gravel filters have such a large exchange rate with the water column that tabs do little. The nutrients enter the water column almost as soon as they leach out of the tabs.
Note that triple phosphate, pyrite, and plaster of Paris are natural products that have all the micronutrients (save iron) necessary for plant growth. So adding a micronutrient supplement from the garden store is NOT warranted. But if it makes one “warm and fuzzy” add just a pinch of a garden micronutrient mix.
Many are concerned about micronutrient toxicity. These concerns are not warranted by the math. For instance, a level of 10 parts per million of manganese becomes toxic to some plants (“The Physiology of Manganese Toxicity”, Horst, W.J. 1988).
The manganese level in Flourish Trace™ is 85 parts per million in the bottle (8.5 x 10-8). The directions are “Use 1 capful (5 mL) for every 80 L (20 US gallons) twice a week“. The calculations are 5 ml is 0.0013 gallons. 0.0013/20 = 0.000065 (6.5 x 10-5). then (8.5 x 10-8) X (6.5 x 10-5) = 55.25 x 10-13 . This is a concentration of manganese in the aquarium water of 0.0000055 parts per million per application. The math become 10 divided by 0.0000055, which is 200,000. So to get to 10 parts per million one would need to dose 200,000 times without a water change. Not exactly something to worry about.
The same math applies to using products such as a garden micronutrient mix. The levels of micronutrients are just too small to be a concern.
Mix the ingredients for both tabs thoroughly, adding just enough water to make a very thick putty (the consistency is that of a drywall compound). Put it into the smallest gelatin capsules one can buy on the internet. These are size 4 gelatin capsules. The filling operation is not easy. The tabs are small and difficult to fill.
Note that these gelatin tabs are composed of animal protein (cooked down hides, hooves, and hair). If you have fish much over two inches in size these fish will smell the gelatin and uproot the tabs. So, if you have larger fish, it is best to make a thin slab of the Plaster of Paris mix and just cut it up into small cubes before it hardens. Indeed, if you don’t want to go through the pain in the butt process of filling the tabs, just do the slab and cut method.
Note that ground pyrite is interesting. It is a mineral (“fool’s gold”) that is attacked by certain bacteria. The bacteria slowly break it down into ferrous sulfate. This slow breakdown feeds the plants the tiny amounts of iron they need constantly. But it isn’t available in some countries and it can simply be skipped with little impact. A good alternative is finely shredded steel wool.
Push the tabs as deep into the substrate as you can. The rate at which the tabs dissolves depends on three elements: the depth they are placed at, the particle size of the substrate, and the total dissolved solids TDS) of the water. The deeper the tabs are placed, the smaller the substrate particles, and the higher the TDS the slower they will dissolve. They might dissolve in a few weeks in shallow placement in large grained sand substrate and soft water. They might take six months two inches deep in a fine sand substrate with hard or salty water.
When providing plants with calcium it is always important to include some magnesium (3 parts calcium to 1 part magnesium is ideal but the above formula doesn’t work too well at this ratio). Calcium and magnesium compete against each other in plant uptake so supplying calcium without magnesium is always a touchy thing to do. Magnesium is a necessary ingredient in chlorophyll, which is important to all plants.
The calcium will slowly make the water hard. We do recommend water changes of 25% once a month to 50% once every two months in any aquarium with light fish stocking. These water changes will keep this hardness buildup controlled.
Other Plant Tabs
Some E-bay entrepreneurs put garden Osmocote™ products into gel caps and sell them as aquarium fertilizers at ridiculous markups. If you want to go that way, buy both the gel caps and Osmocote™ and make your own. These Osmocote™ tabs are a very good choice for placing deep into the substrate under the plant if you do not want to go through the hassle of the plaster of Paris tabs.
Note that these gelatin tabs are composed of animal protein (cooked down hides, hooves, and hair). If you have fish much over two inches in size these fish will smell the gelatin and uproot the tabs. So, if you have larger fish, it is best to make a slab of Osmocote™ in Plaster of Paris and just cut it up into small cubes before it hardens.
Also, note that there are pills for both iron and phosphates sold as “supplements” for human consumption in Walmart. The phosphate pills are very good low-cost options for putting into a fine substrate in a planted aquarium. They are much cheaper than commercial aquarium plant tabs. But the iron tablets have too much iron in them. Do not use iron tablets.
A Note of Caution
Here is a comment from one of my readers:
“I also attempted to make the homemade fertilizer using the second formula with urea, ammonium sulfate, and potassium sulfate. I’m not sure if I should mix it into the water first then add it to the tank, and I’m not sure how much to use for a 10-gallon planted tank that hasn’t matured yet. It’s been about 3 months and I’ve only been using Flourish tabs up until everything came in 3 days ago. only about a quarter of the substrate is planted and there are some floating plants, so I used a cereal spoon and just added 3 scoops. one on the left side, one in the middle, and one on the right side. A couple of days later I woke up to a bacterial bloom. one of the fish that died was a black molly but it had red sores all over it that weren’t there yesterday, and the clown pleco decided to float at the top upside down. It’s still alive because it will randomly take off to another part of the tank every so often, but I’ve never seen it do this before. it also looks slightly bloated, but I’ve only seen it on the glass or some plants, so I don’t know if it’s normal or if it’s new. She has gotten wider as she got older, but she looks bloated in the stomach area, not the sides. on top of this, the whole tank has an awful smell that wasn’t there yesterday. A week ago it had a very weak smell, and I had to be very close to it to notice it was there. Now it’s like a sweaty gym bag. Is it possible I added too much ammonium or that there wasn’t enough CO2 in the water for the plants to use everything so the bacteria got it first? or do you think this was an issue that was building over the last week or so because of something else?”
I felt horrible when I got this message. He added “three scoops” to a ten-gallon tank?? I did not explain things well enough, especially if by “scoops” he means dry fertilizer salts. But even if he only added three teaspoons of the solution he added easily 38 times the dosage for a very well-planted aquarium and 380 times the dosage for a sparsely planted aquarium. And he killed his fish, which makes me feel very guilty. When I got this I rewrote this article about three times, hoping to prevent such a disaster by making this easier to understand with clearer instructions.
The Math for Soluble Fertilizers for the Water Column
The math for the ammonium carbonate becomes:
These are the water-based fertilizer solution calculations for the 0-0-47 Potassium bicarbonate:
Some of the carbonate was replaced by potassium sulfate to lessen the carbonate pH rise without creating solubility problems.
Note ammonia salts all have problems:
- Ammonium carbonate will raise the pH slightly with each addition
- Ammonium sulfate will react with potassium bicarbonate to form potassium sulfate which will precipitate out of solution.
- Ammonium chloride can produce chloride toxicity in plants
- Urea will foster bacterial growth in the fertilizer bottle
So ammonium carbonate becomes the lesser of the evils. But one needs to monitor pH when using it. The pH should not go above 8.2 first thing in the morning in a planted tank. Note the corresponding potassium salts have similar problems. If the pH rises above 8.2 pH with the carbonate formula one must go over to the following for the nitrogen/potassium fertilizer:
- 340 grams potassium nitrate (NPK 13-0-44)
- 5 grams potassium sorbate preservative
This removes the advantage of the ammonium but retains the advantage of the root tabs.
Repeating myself, it should be noted tabs only work with substrate less than 2 millimeter in diameter. Gravel or an under-gravel filter will leach the tab nutrients into the water so fast that there is no benefit to the tabs.
An Easy Alternative
An easy way to regulate all this math is to simply pick the desired nitrate level and add fertilizer accordingly. For instance, measure the nitrate level once a week. If the nitrate level is above 20 ppm, do not add fertilizer for a week. If the nitrate is less than 20 ppm, add fertilizer per one of the schedules above. Easy! Or try 10 or 40 ppm. Some even go for 80 ppm.
Other Fertilization Programs for High Tech Planted Aquariums
It is useful to compare the above fertilization program with what various others recommend. Here are three common methods of fertilization.
Probably the most popular method of fertilization of high-tech planted aquariums is the EI method, originated by Dr. Tom Barr. For most planted aquariums Dr. Barr recommends nitrate nitrogen fertilizer at a level of 3.4 nitrogen, 1 ppm phosphate (P2O5), and 12 ppm potassium (K2O), NPK of 3.4-1-12, per week. For a low-tech tank or low plant mass tank Dr. Barr recommends the levels be brought down to only 20% of this level (0.68 ppm nitrogen, 0.2 ppm phosphate, 2.4 ppm potassium per week). Dr. Barr says the high-tech level can even be doubled for an aquarium with a very large mass of thriving plants.
Aquarium Co-Op Easy Green™ (NPK of 2.7-0.5-9) uses nitrate nitrogen for a total addition per week to a low-tech aquarium of 0.7 ppm nitrogen per week, 0.1 ppm of phosphate, and 2.3 ppm potassium. Easy Green doesn’t have a dosage for high-tech tanks but if one multiplies by five one gets 3.5 ppm nitrogen per week, 0.5 ppm of phosphate, and 11.5 ppm potassium.
For a high-tech planted aquarium with CO2 injection and high lighting levels, some hobbyists measure the nitrate level once a week. If the nitrate level is below 20 they add 10 ppm (0.38 grams x 10 per 100 gallons = 0.38 grams per ten gallons) of nitrate or roughly 0.65 grams per ten gallons of potassium nitrate (NPK of 13.7-0-46). If the nitrate level is above 20 they add 0.65 grams per ten gallons of a potassium salt other than nitrate. This is the equivalent of 2.8 ppm nitrogen and 6.6 ppm potassium added once a week. They depend on fish food to supply phosphorus and iron.
All three of these methods work just fine. But there is considerable research out there that says ammonium is twice as good as urea and six times better than nitrate when growing submerged plants. So I much prefer ammonium fertilization. And I much prefer to put my iron and phosphate into the substrate.
Research has shown a level of potassium roughly three times greater than nitrogen is preferred by vascular plants growing in water and all these fertilizer programs reflect that research. The homemade fertilizer above has nitrogen and potassium levels that are very close to what all these other programs recommend.
Calibration for Fertilizer
Adjusting the water column fertilizer amounts in a well-established aquarium with a large mass of plants is pretty easy. One wants to add sufficient ammonium that roughly 0.5 ppm ammonium is measured two hours after adding the ammonium fertilizer. If it is more than 0.5 ppm, drop the amount of the fertilizer added. If it is below 0.5 ppm, add to the amount of fertilizer added. In a planted aquarium with new planting, one has to just wing keeping the ammonium lower than 0.25. But note one will need to calibrate your test kit. Some test kits read 0.25 with distilled water.
There is one situation where a fertilizer of sorts is called for in all types of fertilization. If the water is very soft (dGH less than 3 or GH less than 72) then it needs both calcium and magnesium additions for most plants. The easiest way to do this is to simply add one level teaspoon of Epsom salts (magnesium sulfate) and one level teaspoon of Plaster of Paris (calcium sulfate and calcium carbonate) to every ten gallons of water change water.
The Science behind Fertilizer Chemicals
The “NPK analysis” found in all fertilizers is a series of three numbers (like 13-3-15) which indicates the percentages of nutrients in the fertilizer. The first number is nitrogen (N). The second number is phosphorus (P or actually P2O5). And the third number (K or K2O) is potassium (“K” is from the Latin word for potassium – kalium).
Here are the chemicals we are using above:
Ammonium sulfate is the chemical formula (NH4)2SO4. It has an NPK analysis of 21-0-0
“Triple Phosphate” or “Super Phosphate”. This chemically is calcium dihydrogen phosphate or monocalcium phosphate, [Ca(H₂PO₄)₂ .H₂O]. The NPK analysis is 0-46-0
Potassium sulfate is K2SO4. It has an NPK of 0-0-55
Potassium nitrate is KNO3. It has an NPK of 13-0-44
Ferrous sulfate is FeSO4 or “Iron sulfate” and is used to supply iron to the planted aquarium.
Pyrite is useful for supplying iron in very slow amounts to plants. It is made for use in jewelry.
Epsom Salts are just magnesium sulfate. It is available in any drugstore for making a solution to soak feet. Magnesium is normally available in most waters at adequate levels but needs to be added if one is adding calcium.
Plaster of Paris is chemically just Gypsum (CaSO4-2H2O) which has had 75% of the water driven out. It typically has up to 40% calcium carbonate (ground limestone) in it as a cheap filler. When water is added to the powder the plaster becomes a hard solid material. It is widely used in arts and crafts hobbies. It is ONLY used here as a binder, it is not a fertilizer per se.
Fertilizers in More Depth
We go into aquarium fertilizers in more depth in the following links:
15.5.5. DIY Epiphytic Fertilizer
15.5.6 Fish Food as Fertilizer