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Translated in Dutch by Joost Abrahams

2.15. Cycling a Planted Aquarium

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Author : David Bogert

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Time To Read :
6 minutes
Difficulty : Level 6

Excerpt :

It is difficult to cycle a planted aquarium, requiring a HUGE amount of patience.
Cycling a planted aquarium is tricky. If the plants start growing from the get-go the plants can absorb the ammonia before the beneficial bacteria can get to it. Now, this isn’t completely detrimental. The plants will simply act in place of the beneficial bacteria, and all will be well. There will be short spikes of ammonia during darkness but they will be relatively harmless to fish.

But normally newly planted plants die back and don’t absorb ammonia for a while. The soil balls around some of the plants can also release ammonia for a long time. Depending on the setup the plants can take weeks or months to start absorbing the ammonia. So things can get complicated.

There are three possible scenarios here. These include, in order of preference:

  • Cycle with no plants and no fish
  • Cycle with only plants, no fish
  • Cycle with plants and fish

All three of these can be done quite easily and quite successfully.

Low tech Planted Aquarium
Low tech Planted Aquarium

Note there is a challenge here. Cycling is a natural process and Mother Nature is very unpredictable. Plants are decidedly more unpredictable than fish, so the outcomes and courses of events to be expected are quite simple: Don’t be surprised by ANYTHING that happens. It is ALL VERY UNPREDICTABLE.

One can have five new identical aquarium setups with plants side by side. One can give each aquarium EXACTLY the same treatment. And each tank can do something completely different. That is just Mother Nature at work. You will notice I do NO experiments with planted aquariums in my write-ups. That is because planted tanks are so unpredictable, I would need twenty identical tanks to come to any sort of meaningful conclusions.

Low tech Planted Aquarium
Low tech Planted Aquarium

Also, plants do not respond anywhere near as quickly to something as do fish. Plants normally take six to ten weeks to “settle in” and send out new shoots. When we talked about cycling a fish aquarium, we mentioned three things were needed:

Patience, patience, and patience

When cycling an aquarium with plants the three things needed are:

Huge amounts of patience, huge amounts of patience, and huge amounts of patience

Low tech Planted Aquarium
Low tech Planted Aquarium

Cycling with No Plants and No Fish

If an aquarium which is to eventually be a planted tank is cycled with no plants and no fish the whole thing can be done by any one of the many methods used for fish aquariums with no fish present. This is decidedly the preferred method of doing this. If one gets some good beneficial bacteria going in a filter before adding the plants, things will go much easier down the road.

The many, many ways to do fishless cycling in any aquarium can be found in these links:

2.1. Standard Fish-less Cycling

2.2. The Many Ways to Cycle a Tank

2.3. How I Cycle a Tank

2.4. Cycling with Ammonia

I personally cycle without plants in a darkened aquarium for at least 6 weeks to get things started but that isn’t a necessity.

Low tech Planted Aquarium
Low tech Planted Aquarium

Cycling with Only Plants, No Fish

This can be a very frustrating way to cycle a planted aquarium or it can be a very stress-free way to start up a planted aquarium. Sometimes plants take off and don’t die. Sometimes plants start dying as soon as they are put in the aquarium. And sometimes plants take off initially and only start dying in a few weeks or a few months. It all depends on things like CO₂, lighting, plant selection, and substrate. And it is all VERY unpredictable. So ammonia absorption becomes very unpredictable

Newcomers to the hobby obsess over every detail of the process and keep trying to “figure it out”. They come upon this website and ask things like “I’ve been cycling my planted tank for 8 weeks now. Why isn’t my ammonia dropping”. And my answer always is pretty much “damned if I know why!” A cycling planted tank can do just about anything. It is completely unpredictable.

Walstad Planted Aquarium
Walstad Planted Aquarium

So to cycle a planted aquarium just set up your planted aquarium by the method you have selected. Add the plants. Turn on all the equipment (lights for 4 hours at a time, CO₂, etc.). Then simply cycle the tank using any of the methods found in these links:

2.1. Standard Fish-less Cycling

2.2. The Many Ways to Cycle a Tank

2.3. How I Cycle a Tank

2.4. Cycling with Ammonia

The only thing that changes with ALL these methods in the definition of “end of cycling”. “End of cycling” in a planted aquarium becomes when the new plants get established and start putting out new leaves and new shoots AND ammonia falls to less than 0.25 ppm 24 hours after adding some ammonia. After the new leaves or shoots start appearing and the ammonia is being processed, stop any additions, do a 95% water change, and add any fish one wants to add.

This cycling period can be anywhere from two to twenty weeks. Sometimes algae will take over and one will need to start from scratch. The only constant here is that there never is a constant.

Low tech Planted Aquarium
Low tech Planted Aquarium

Cycling with Plants and Fish

When one sets up an aquarium with both plants and fish in the aquarium one MUST treat the tank as though it has no plants in it. One MUST do a fish-in cycle or no cycle at all. Simply feed lightly for at least two months and see where everything goes. Measure the parameters four hours after the lights come on and do water changes if anything gets into the yellow “Alarm” levels below.

pH of the waterAPI Ammonia test in ppmAPI nitrite test in ppm
pHAlertAlarmToxicAlarmToxic
6.616641600.52
6.811441100.52
7.08328014
7.2520501.56
7.43123028
7.6282028
7.814102.510
8.00.838312
8.20.6263.514
8.40.4243.514
8.60.2512416
Nitrate alarm level is greater than 80 ppm.
Nitrate toxic level is greater than 440 ppm.

With both plants and fish in an aquarium, the aquarium can be cycled by two different methods:

2.5. Fish-in Cycling

2.6. Not Cycling at All

Low tech Planted Aquarium
Low tech Planted Aquarium

Testing Water Parameters and Definitions

Decomposing plants and fortified soils put out ammonia on a 24/7 basis, i.e. constantly. Plants only absorb ammonia when bathed in light. So the timing of testing water parameters becomes somewhat tricky. It is very common to have very high ammonia when the lights are turned on in the morning. Then by the end of the well-lit period, the ammonia is gone, absorbed by the plants.

So everything starts boiling down to a semantic problem. What is the definition of a “cycled planted aquarium”? I would define a planted aquarium as “cycled” when the ammonia is less than 0.25 ppm first thing in the morning when 1 to 2 ppm ammonia was added the day before when the lights were turned off. But that is completely arbitrary.

Also, note that nitrifying bacteria oxidize ammonia to nitrate on a 24/7 basis. So even temporary ammonia spikes are avoided with nitrifying bacteria. This is why I recommend doing a fishless and plantless cycle for all aquariums on set up.