Wise old fish

I've had good results with spider plants, Chlorophytum comosum. They must be planted with leaves above water, or they will rot, But - if correctly planted they develop huge under-water root balls with vigorous growth of leaves and "spiderlings' . It's easy to harvest and remove biomass - just snip off the large spiderlings, and re-home in other tanks if needed. My rule of thumb if that plant growth (biomass) is vigorous, then by definition they will be removing nitrogen from the water and you're doing the right thing. Terrestrial plants from what I can find are about 3% nitrogen by weight.