And then there's water chemistry. We seem to have awfully hard water with a high pH. The pH has been pretty consistently around 8, the alkalinity between 80 and 100, and the hardness above 150. Since I started checking it the ammonia levels have been low, but the nitrite levels are still in the "dangerous" range - around 5 ppm. Nitrates are around 20, which is ok. I suppose that if NH3/NH4 and NO2- are stable, that means that all the added N will either end up as NO3- or plant biomass (i.e., until I do a water change).
Although the NO2- levels are still "dangerous", the fish seem happy enough. High levels of nitrite reportedly will damage gills, but the fish seem to be able to cope with current levels. As long as they don't get any higher I suppose I should wait a while to change the water - presumably, the Nitrobacter will grow faster the more substrate there is for them. According to this site, Nitrosomas and Nitrobacter do best at a pH between 7.4 and 8, so I suppose there's some good to be said for the pH of the water. (Of course, I have no idea whether that website is a reliable source of info or not, and I'm too lazy to figure that out).