11-10-2009, 02:03 PM
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There are a number of variables involved in this issue, and no hard and fast numbers. My response may be lengthy, but I hope it will answer your questions and be explanatory.
Plants are better filters than any equipment you can put on an aquarium. It is technically possible to almost have a closed system. For many years before we had lights, filters and heaters, aquarists maintained healthy aquaria with fish and plants, and never performed a partial water change. But the success or failure of such a system is biological; the system must be balanced between fish, plants, invertebrates and the bacteria. Heavy planting with very few fish; I read one author who mentioned 6 or 7 neon tetras in a heavily-planted 55g aquarium would work. Most of us want more fish in our tanks.
With adequate mechanical filtration and regular maintenance, a heavily-planted aquarium can support more fish than one without plants--but only provided all else is equally balanced. Not all fish work in this scenario. A 55g tank with 150 small characins that are compatible, that is heavily planted, that receives a weekly 50% partial water change, and with mechanical filtration having minimal water movement, will be successful. Replace the 150 characins with a few oscars and it will not.
"Compatible" means the fish and plants share the same basic requirements with respect to water parameters (hardness, pH, temperature) and environmental things like wood and rocks, as well as being compatible in behaviours and temperament. Fish are less stressed in an aquarium that replicates their natural environment as closely as possible. And less stress means better health.
No filter (equipment) can remove the toxins that accumulate in all aquaria. Fish produce solid and liquid waste regularly; the bacteria breaks down the solid that is trapped in the filter or accumulates on and in the substrate. But the resulting liquid plus the urine remains in the water until it is removed through a partial water change. Plants can remove these toxins but it is a slow process and only works when the fish/plant/water ratio is something like that I mentioned previously. This is why in planted aquaria more filters do not improve water quality. Plants and bacteria keep the water clean, mechanical filters keep it clear--two very different things.
With respect to the nitrification cycle, in a well-planted aquarium plants will use more of the ammonia/ammonium than the bacteria, and because plants are faster at using it, the nitrification bacteria are few in a well-maintained planted aquarium. Since many of these aquaria are soft acidic water setups, and it has been shown that the nitrification cycle basically stops when the pH is at 6.0 or lower because the nitrosomonas and nitrospira bacteria are unable to multiply and even survive in acidic water, it is obviously not the bacteria that are consuming the ammonia produced by the 150 fish in the 55g aquarium, but the plants.
Of course, the danger if one takes this too far is the always-present risk of something going wrong that upsets the biological equilibrium with disastrous results. So, in my view, while one can theoretically maintain more fish in a planted aquarium, the practicality of this working depends upon everything being balanced; and this delicate balance requires competency on the part of the aquarist to ensure long-term success.