I have a 55g with LR and a few inverts for cleaning. Over the last 8 months I have introduced a variety of fish, only to have all die, except this blue damsel which I have now had for 6 months. I've tried QT and doses of formalin, malachite green, Rid-ick, osmosis shock, but one by one they start their own private party in the corner and are rigered shortly after. Water params are fine, no ammonia, nitrate, nitrites, etc. SG 1.023. The only fish that has survived it all and been through all the treatments is the blue damsel. The inverts seem unaffected by whatever is the issue. I have not seen white spots, black spots, protruding scales, bloating. They swim normally and eat normally, but just within 24-72 hours of changing behavior they will crash and die. Any ideas?