sorry to hear that.
there are 3 things i observed, but already been noted by someone else.
1) Sandbed. i would not do it next time, that way you can vaccumm out the dead tissues they split out. sandbed in treatment tank can do alot more harm than good. bare bottom is best.
2) I always have doubt about the dosage and the purity of the med that is made for animal. I would use only med for human.
3) prob not that important but the heater in the plastic container is a no no for me. those plastic are very very thin and does not take much to heat it and melt it. You ever touch those heater when they are on? they are actually pretty hot. so it can release some chemical into the tank. just my 2 cents.
good luck next time.