Hello, I could use a little help figuring out why my fish have been dying. I lost a neon yesterday, and today I lost a platy. There is another platy in the tank that seems to look a little different on his back directly behind the dorsal fin. The area is not shiny like the rest of his body, and it almost appears like a few scales are sticking out a little bit. No other fish in the tank appear ill. I had recently tried to unsuccessfully introduce a few other neons into the tank (as I only had 2 and wanted to bring in a few more) but they all perished at different times. Those had some tail fin issues, and it seems like those fish arrived sick from the store without me noticing (I'm not happy that a pet store would sell sick fish!!) I'm wondering if maybe one of them introduced something into the tank? The tail fin issues seemed to be like they had been bitten off in places. My pH is 7.0 ammonia: 0 nitrite: 0 nitrate: 10 What should I do? Other fish in the tank: 1 neon tetra 2 platys 2 mollies 4 glow tetras 1 ghost shrimp I treat the aquarium (20 gallon tank) with Prime.