I set up my new 90g tank about 5 months ago. I have sand, a coral rock and a piece of driftwood in there. I had it running with the filter for about a month and then I added a few feeder fish. Within a week they all died. I kept the tank running for another month doing a pwc of 20-30% every two weeks. I then added some tiger barbs n a catfish n they were fine. When I did the next pwc they started dying about an hour later. I checked the pH and temperature and it was fine. The water I put in was the same as the one already in there. The. I started getting a lot of diatoms even though I started doing weekly pwcs. After two months of that without fish I did a 50-60% pwc n the diatoms stopped. I've had live plants in the tank for about 6 weeks now. The pH is at 7.6. Ammonia nitrate and nitrite are at 0. The water is soft. These parameters have been constant for over a month. I just put in some new fish. A few goldfish. A few mollies. And two plecos. One of the mollies already died. Another is floating at the top. And one of the plecos is not looking good. I don't know what to do