How long have you had it? Any lighting changes recently? By “parameters are normal” I’m going to assume they are at or slightly above NSW levels so nothing is wrong in that department, but others will want to see numbers. My guess would be something along the lines of too much light or an alk/pH spike (from missing lots of doses and then doing a big dose all at once).
Lighting hasn’t changed with my radions in prob 2 years. Idk. I’ve got an aquarium guy coming out Thursday to give his assessment. Will do the best I can till then.