I bought 4 ghost shrimp on Monday. I had 1 in the tank already. Thursday night when I checked on the tank they were all fine. Friday morning all 4 dead, but the original one was still kickin. I had been treating the tank with a low dose (1/4 cap) of Maracyn Oxy because one of my baby angels had fin rot (all gone now), I treated Tues and Wed. That particular med doesn't require you to remove activated carbon. Also I fed the fish in that tank some frozen brine shrimp Thurs night. The levels are fine, I checked Thurs night. Ammonia was 0, Nitrite 0 Nitrate 20ppm (before water change). Temp is at a steady 78. Tank dwellers are panda cories, otto cats, 2 scissortails and 2 tiny angel fish (there's a snail somewhere in there too that hitched a ride on my water-sprite lol). Any theories on the culprit???