I have a 16 gallon tank tank set up on March 4th. Tank was cycled with fish, prime and bottled bb. The first fish in the tank were 3 dwarf Cory's and an opaline blue dwarf gouramI who were getting an upgrade from a 6 gallon. Last water test were: 0 ammonia, 0 nitrites, around 40ppm nitrates. I have a moderately to heavily planted tank, so I'm not sure why my nitrates aren't dropping. First new fish added were 5 variatus platys. They actually seemed to be struggling the bag when I got home, but I added them to the tank anyway. 1 died within hours. 1 died in a week. Then they all started hiding. Another died in a month. The next died after 6 weeks. I have one variatus left and he's still hiding. About 2 weeks after I added the variatus my 4 month old dwarf gouramI died. 10 days ago I decided to try some guppies, 1 male 2 females. One female looked a little bad from the start. She was hiding a lot and looked weak. She did come out the last few days before she died and swam with the others. I thought she was looking better but died a couple of days ago. I have never had this much trouble keeping fish alive. The fish I still have in this tank and another are several years old. What is causing these deaths. I'm afraid to buy any more fish until I sort this out.