I've had two rainbow sharks die on me in the past 3 weeks. I bought one on the 16th and added it to my established 75g. A week later it died, so I figured it was a fluke and brought it back to the store and got a new one. About a week and a half later this one started acting funny too and it was dead the next day. All the other fish in the tank are fine (3 yoyo loaches, 6 upside down catfish, 5 turquoise rainbows, 2 bristlenose pleco). My parameters are 0 ammonia, 0 nitrite and 20 nitrate as tested with API master test kit. It's a natural planted aquarium with a large piece of driftwood. Have I just gotten a run of bad luck or are these really that difficult to keep alive?