Interesting read. I had added an airstone thinking that maybe lack of O2 was part of the problem, but that's the opposite of what I should be doing! The water flows from by sump up to the duckweed tank and then overflows into the FT. The water is relatively still, but there is a high exchange rate. I'll have to setup a different, stagnant tank with some of the duckweed and see if it fares much better.