I experimented with an el natural tank a while back and it ended up with major algae problems. After starting a water change/fertilizing regimen, the algae lessened substantially. I have also noticed in my other tanks that if I slack off on water changes, the first symptom I will see is algae growth of one kind or another. I'm wondering why this would be, I was thinking that it was due to the buildup of organic matter and thus somehow fueling the algae growth, but I can't seem to find an answer in more specific terms. One idea I've had is that the organic waste breaking down in the substrate, is releasing more ammonia and thus providing a constant low level of ammonia to the tank, which according to your logic above, would do much more to fuel algae than the plants. Are there any studies that you are aware of, that show what plants do as far as ammonium uptake, at very low levels (nearly undetectable)? Is it possible that plants will actually not use ammonia at all at levels of around 0.1 or 0.2....and therefore the water changes were both reducing ammonia in the water, plus reducing the waste generating ammonia, and thereby reducing algae problems? But then, the question that would bring up, is that in a cycled tank, any excess ammonia that waste in the substrate is producing, should be cycled into nitrate fairly quickly as the bacterial colony expands to accommodate this gradual change. So that theory doesn't seem to fit both cases.