You are probably right. But there are so many unknown factors that are difficult to account for.
Although ammonia is excreted from fish in several ways, Ammonia in the aquarium cannot just be spontaneously created. There needs to be a source of nitrogen into the system. The only way that can happen is from food. Nitrogen gas is insoluble in water so it can't come from the air.
In every aquarium, there is much more nitrogen fed to the fish than the fish is going to convert into new proteins and tissue. That is where ammonia comes from. The biological filtration will convert the ammonia into nitrite and then nitrate. Nitrate is much more difficult to metabolize which is why most aquariums accumulate nitrate over time.
Plants and algae can absorb some nitrate from the water. But it does not appear as if this tank is loaded with plants and algae.
The other way is anaerobic filtration. Anaerobic bacteria use nitrate as a source of oxygen and convert it to insoluble nitrogen gas.
If there is a good anaerobic filter situation running it could explain the low nitrate levels. Anerobic conditions can arise within a canister filter that is clogged, a gravel bed that has been undisturbed for a long time, or a sponge filter with a very slow flow rate. The problem with anaerobic filtration is that it's not very stable and prone to going anoxic. When that happens the bacteria will then begin to convert sulfate to hydrogen sulfide in order to get oxygen. Hydrogen sulfide is toxic and can poison the aquarium.
That's why changing the water, maintaining the filters, and vacuuming the substrate is so important. It dilutes nitrate, and prevents anaerobic and anoxic conditions from forming.
Andy
