What we have here is called root rot. You may be able to fix it with H2O2 if you add it every other day. It may be too late though. Your problem is most likely res temp is too high, keep it lower than 75F.
i was told that it wasnt root rot cuz there was no smell,so i managed to drop the temps and they picked up a little,so i giving them some root treatment,any more suggestions
