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15.3. Fish Limitations

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Author : David Bogert

Published :

Time To Read :
7 minutes
Difficulty : Level 6

Excerpt :

It is challenging to have even moderate amounts of decent sized fish in a planted aquarium.
I’ve seen thousands of aquariums with lots of fish in them and hundreds of aquariums with lots of plants in them. Notable almost absent from the vast majority of these thousands of tanks are tanks which have both large amounts of plants and moderate amounts of medium sized or large fish. It is just VERY difficult to have such an aquarium. BUT there have been some who have succeeded. Mother Nature is very forgiving.
planted aquarium
planted aquarium

The “Fish Problem”

When one combines a moderate amount of fish and plants in an aquarium there typically will be problems. It has to do with aeration. Moderate amounts of fish in any aquarium REQUIRE good aeration to stay healthy. Good aeration very rapidly insures that the oxygen level in the water is high because oxygen is present in the atmosphere at a level of 220,000 parts per million. But good aeration does not rapidly replenish carbon dioxide as carbon dioxide is only present in the atmosphere at 4,000 parts per million.  Plants feed on carbon dioxide. So in a well aerated aquarium plants basically will just “starve” and die.

The CO₂ level in a well planted tank can rise to 3 ppm overnight as 2 to 3 ppm CO₂ is the “equilibrium” point for CO₂ in an aquarium. .Then, in an aquarium with aeration by surface agitation or air bubbles, within one or two hours of turning on the light it can fall to “undetectable” and the plant growth stops. Even with good aeration the CO₂ replenishment from the atmosphere will not keep up with heavy plant growth metabolism in good light.

This is why CO₂ injection works so well in a planted aquarium. But then CO₂ injection doesn’t work well in a well aerated tank as the CO₂ gases off to the atmosphere with heavy aeration. And, if there are a lot of larger fish in a tank which is not well aerated, oxygen levels can drop from 8 ppm to 3 ppm (typically overnight in a well planted aquarium) and the fish can die.

Aquarium with plants and lots of small fish
Aquarium with plants and lots of small fish

There are several very general, frequently not followed, recommendations for a planted aquarium:

  • Light stocking of fish smaller than two inches (5 cm)
  • A carbon dioxide CO₂ level above 10 ppm first thing in the morning
  • A plant every three inches across both the length and the width of the aquarium
  • Little surface movement of the water and no air stones.

Note that fertilization is NOT included in this list. Fertilization is MUCH less important than many seem to think. That may have something to do with the obscene profit margins on various “aquarium fertilizers”.

Low Tech Planted Aquarium
Low Tech Planted Aquarium

The thing to key in on is the first recommendation: light stocking of fish smaller than two inches (5 cm). This is where most hobbyists make their mistake.

Many aquarium hobbyists start out with both plants and large numbers of sizable fish in an aquarium. Then their plants start dying or their fish start dying and they keep replacing them and trying all sorts of thing to keep them alive. Finally, after a few months or even a few years of fighting Mother Nature, they gravitate to either a planted aquarium with few fish or a fish aquarium with no plants in the aquarium.

Many hobbyists start out with something like a twenty gallon tank with five plants and five platies, two angelfish and five swordtails. They have an airstone in the tank providing aeration. And the plants all die over a span of two to four months. Or there is little aeration of the tank and the plants thrive and the fish die. This is very common.

planted aquarium
planted aquarium

The major challenge of keeping a lot of fish with a well planted aquarium has to do with aeration. In nature there is only a very light stocking of fish in any waterway, like the equivalent of a few guppies in a fifty gallon tank. And the ratio of surface area in the waterway to the mass of the fish is very large, meaning the fish get plenty of aeration in nature even if the surface area has no “choppy waves”.

So when a beginner puts several three inch fish in a twenty gallon aquarium it becomes an unnatural situation. The ratio of surface area to fish mass is low. This is a situation where artificial aeration (choppy waves or bubbles) is required in order for the fish to thrive. Artificial aeration is bad for plants, very bad for plants.

If an aquarium has a moderate stocking of fish, the moderate number of fish use up the oxygen at a rapid rate. There thus must be a way to get oxygen quickly into the water. When the oxygen is added with lots of aeration, the carbon dioxide in the water rapidly disappears. Most plants need high levels of carbon dioxide in order to really “thrive”. Thus the challenge. Note there are some plants like anubias, Java fern, Java moss and swords which grow anywhere and in any situation.

Fish aquarium
Fish aquarium

Moderate amounts of both fish and plants is a situation which requires expensive carbon dioxide injection systems. The hobbyists must put in carbon dioxide to a level where the plants thrive but not to a level where the fish die. AND one has to aerate the water well to keep the oxygen levels up. Aeration removes carbon dioxide. So one goes through carbon dioxide cylinders rather rapidly. This is NOT something a beginner wants to tackle.

Most cichlids also present a problem. Large American cichlids and all African cichlids just love tearing up and uprooting plants. The only cichlids I’ve seen in planted aquariums are discus, angelfish, rams and apistos.   I was wrong!

What African cichlids do to plants
What African cichlids do to plants

Now after I wrote what was above I found this aquarium on social media:

Malawi Cichlids and Plants
Malawi Cichlids and Plants

There is some vallisneria and a lot of Amazon swords (Echinodorus amazonicus and E. bleheri). This tank is a year and a half old with crushed lava rock substrate and a pH of 7.0. The fish have been in it the whole time and are decidedly overstocked, which I would have said was completely and totally impossible. Gorgeous and amazing! So much for my pale attempts at a planted tank with Africans. AAARRRGGGGHHHHHH………….

What are Acceptable Stocking Amounts?

A fair question is “What is the maximum number of fish  I should put in a planted aquarium? We calculate a “light stocking” in several articles on stocking. These are the MAXIMUMS calculated, with the clear understanding that these are NOT rules and only guidelines (i.e. opinions and ONLY opinions) which can be violated if you want to:

Stocking a Planted Aquarium
Stocking a Planted Aquarium

What this means is that one can put in a five gallon planted aquarium a maximum of twenty 1 inch pigmy rasboras, a maximum of four 2 inch platies or one 3 inch betta. You  should not put a 4 inch gourami in a planted five gallon. Easy!

Now the caveat here is that this is the loading of fish that can survive without supplemental aeration. This is important for a planted aquarium. Most good planted aquariums have much lower levels of fish stocking than even this light stocking.

planted aquarium
planted aquarium

Planted Aquariums in Depth

The following sections will tell you how to make a planted aquarium:

15. Planted Aquariums

15.1. Planted Aquariums in Depth

15.2. Fish for Planted Tank

15.4. Types of Planted Aquariums

15.5. Fertilizing

15.6. Carbon Dioxide

15.7. Substrates for Planted Aquariums

15.8. Walstad Aquarium

15.9. High-Tech Planted Aquarium

15.10. Hybrid Planted Aquariums

15.11. Many Fish Many Plants

planted aquarium
planted aquarium