Yes, plants are part of the picture. I figured that the filter bacteria wouldn't be contributing a huge amount of CO2. The only benefit I can see in biological filtration is that there are more sites for bacteria to colonize and more Nitrogen can be processed. And sponges, etc., are good for filtering out particulate matter. But I'm not sure how big or small the difference in Nitrogen processed actually is.
With a fully planted tank, the plants' uptake of Nitrogen should take care of a good part of the Ammonia.
Yes. Many suggest not "encouraging" biological filtration in planted tanks. This is because the nitrifying bacteria are in competition with the plants for ammonia/ammonium. Studies have shown that plants are actually faster at taking up ammonia than the ammonia oxidizing bacteria (
Nitrosomonas sp.). I engaged in a discussion with Tom Barr and others on one of the plant forums a while back, as to how much ammonia plants can take up, and if adding new fish would increase the ammonia to a mini-cycle as can occur in non-planted tanks. Tom said the amount of ammonia capable of being taken up by plants in a well planted tank is very high, and higher than would ever cause issues for fish unless something went very wrong. This has certainly been my experience; I set up new tanks with plants (floating are ideal for this) and fish and I have never had a "cycle" that could be measured.
Many will advocate planted tanks with no filter at all, and provided the planting is significant and the fish load moderate, there is no question that this works with no problems. So further proof that biological filtration need not be "encouraged." Obviously, nitrifying bacteria will colonize surfaces in the aquarium, so the presence or absence of any biological "filter" is not going to cause nitrogen-related problems for the fish, always provided everything is naturally balanced. I wouldn't overload the system.
Mechanical filtration, such as with sponges, filter floss, pads, etc, is beneficial in my opinion, but here again there are those who say this is not necessary. But having experiemented with a no-filter planted tank, it did seem that truly "clear" water required a sponge filter in a 10g tank. Mechanical filtration removes suspended particulate matter which can settle on plant leaves, inhibiting the exchange of nutrients, oxygen and CO2 that is essential for plants. And such filtration also results in a current which achieves this, plus it brings nutrients to plant leaves, and it provides more uniform temperature. But again, one can have a perfectly healthy natural aquarium without this too.
Nitrite and nitrate should never be issues in a planted tank, unless one or both of these is being artificially introduced via the water changes which is a very different set of issues. But assuming "pure" water from the tap, the ammonia being produced by fish and the breakdown of organics will easily be handled by the plants, though a tad will get taken up by the bacteria. But nitrite will be zero, and nitrate very low (less than 10 ppm is normal, many have zero nitrate).
Byron.