Yea I have sera Florena tabs under there that seem to have broken up and loads of white bits on sand. Don't use them anymore but they are still there somewhere.
Because I’ve been there myself a few years ago – overzealous on my new experience with planted tanks. The first batch of corys died. I did think, indeed, that it was a “bad batch”... Then another... Then another type of corys... then another one... Famous Diana Walstad was kind enough to reply to my email and we eliminated one thing after another: H2O, bacteria and finally – poisoning on ingesting particles of Laterite which I had at the time cupped with very small grade gravel (nearly sand grade). Did you see your fish in the feeding frenzy when they accidentally pick up their tank mate’s poo and spit it out? Well, tiny particles of this poo are swallowed – no harm there. HOWEVER there is harm when a fish swallows a tiny particle of a root fertilizer.Why would Florena tabs kill the fish? That is a bold statement..