I like to use floating plants as a fertilization indicator in my low-tech tanks, because I know that CO2 is never a limiting factor for them. I recently started a 20L tank, planted with various crypts, some bacopa monnieri, compact amazon swords, and some hygrophila spp. For floaters, I put in red root floaters and amazon frogbit. The substrate is about 2 inches of ecocomplete over a thin layer 1/4 inch of peat moss and a sparing amount of osmocote plus.
After a couple of weeks, I can't seem to get the frogbit and the red root floaters to thrive---the roots seem to die, and the leaves turn yellow and start to melt. This goes for healthy frogbit that I move from my other established tank. Likewise, when I move red root floaters to the other tank, they also do well. There is some current in the tank, but I have corralled them so they don't move, and the current is no greater than in my other tank, where the frogbit is doing well.
It almost seemed to get worse when I added small quantities of fertilizers (KNO3, Seachem Equilibrium). I've seen the effects of low potassium on frogbit (waffle pattern browning) and this seems very different. The underwater plants aren't as badly affected, but they so are establishing themselves as well as I think they should, and the bacopa seems to show inter-vein chlorosis on new leaves. I think the same thing that is happening to the floaters is happening to the other plants, but more slowly.
The ammonia was about 0.5 ppm, the nitrite was about 0.25 ppm and the nitrate was about 5 ppm last time I checked on Saturday.
Pictures are attached.
Has anybody seen something like this?