Tom,

It sounds like you test more than I do!

When I finally noticed that one of my long-running low light, non-CO2 tanks had gone into decline, the first thing that I did was to test the NO3 and PO4 levels. I found that the former was close to 0 ppm, from the normal 10 - 20 ppm. The drop was apparently caused by the sudden explosive growth of duckweed, perhaps coupled with the depletion of the soil substrate.

I added enough KNO3 to get the level back where it normally was, removed a lot of duckweed, and the tank is now reovering. The diagnosis and correction took about 15 minutes. I'll check the NO3 level monthly for a while, and pay more attention to the duckweed.

If my NO3 test kit is within 5 ppm or so at normal aquarium levels, that's close enough for me.

When someone with a high light, injected CO2 tank has a problem, he is often advised to do a major water change and redose. That advice seems to usually work, but he never knows which nutrient was low, if any were, and if one was, why it was. That advice, good as it is, can mask the problem.

I prefer to know what is going on in my tanks.

I'm off to the shores of Lake Ontario, to watch the annual salmon slaughter. Killing king salmon is no great evil since they are soon to die anyway, but people also catch steelhead and brown trout and kill them too, and that is a loss.

Bill