Re the nitrate discussion. I'm going to see what happens when the plants are in. Hopefully they can bring the levels down naturally and I won't have to resort to more drastic measures. But I'm in no hurry. I've been preparing my tank for two months already - I figure doing a bit at a time makes it easier, to correct the little problems before they become big ones.
Plants do use nitrate, but this is very limiting. With nitrate at 20 ppm in the source water, you might see it lower than this in the aquarium with fish, but I would not think by much if at all. Remember that nitrate will naturally occur within the aquarium (depending upon the fish load), so this is adding to the 20 ppm in the source water. To make a dent in this, the tank would have to be thick with plants and few or no fish. I have between 0 and 5 ppm nitrate in all my tanks, and they all have plants, including heavy floating which are especially good at using nutrients. I have known aquarists with well planted tanks (low-tech) with zero nitrates, so all of the nitrate occurring within the system was being dealt with by either the plants or denitrification. And the latter is another means of reducing nitrates, but I won't get into that now.
Aquatic plants obviously need nitrogen, as do all higher life forms. But unlike terrestrial plants, aquatic plants generally prefer ammonia/ammonium over nitrite and nitrate as their main source of nitrogen. So in an aquarium, most often the plants will grab the steadily-produced ammonia/ammonium and use it, turning to nitrite/nitrate when ammonia/ammonium is no longer adequate and provided everything else is still sufficient for photosynthesis. The reason they do this is because taking up nitrite or nitrate directly means they must change it back into ammonium, and this is obviously extra work that uses valuable energy, so it is a "last resort" so to speak. Nitrate is taken up directly by a few aquatic plant species, and this plus the rapid use of ammonia/ammonium before the
Nitrosomonas bacteria can get it is why nitrate is usually low in planted tanks. With less ammonia/ammonium becoming nitrite, there is less nitrate resulting from the
Nitrospira bacteria's oxidation of nitrite to nitrate. As photosynthesis slows, the plants' need for nitrogen obviously lessens accordingly.
The faster the plants grow (photosynthesize), the more nitrogen they need. This is why floating plants are so effective. Being at the surface, they have two real advantages over submersed plants--the brighter light, and the aerial advantage. The latter is the uptake of CO2 directly from the air rather than the water, a process which occurs about four times as rapidly from air. So floating plants will take up more nutrients from the water to balance; they are often referred to as ammonia sinks for this reason, which is why I use them to "cycle" tanks rapidly. Light of course is the driving force behind photosynthesis; with light intensity that is sufficient for the plant (each species has its level), photosynthesis will occur at the maximum rate, provided all other requirements (the nutrients) are met.
Byron.
