Originally Posted by Dasieq

I am having some difficulties with my aquarium and I am new at the hobby, maybe someone can help. I recently got a used 29 gallon tank, and I've had it now for a little over two months. I bought a heater, filter, etc and rinsed all gravels and tank decor, filled the tank, I used a water conditioner to remove chlorine from the water, ran the tank with the filter for over a week before adding fish. I brought home two mollies, two swordtails, and two tiny cherry barbs from Petco and I was told they were compatible tankmates. Three of the fish died and it prompted me to have the water tested. The worker at Petco said that the water was perfectly fine with the exception of a little hardness, and to add a little salt, which I did. Since I have started, the water has been crystal clear, never cloudy, I have regulated the temperature between 78 and 80 degrees, kept the filter running 24/7, and have changed the water regularly. Despite my efforts the fish keep dying. I have added fish, and lost them. I have three that have lived for about one and a half months, which is the longest of any of them. Less than a week ago I bought two creamsicles at Petco, and three days later one of them had already died, and the second one is looking like he might face a similar fate. I cannot figure out what I am doing wrong. I have only bought fish from Petco and have never gone anywhere else, so I'm not sure if it could be their fish rather than my own mistakes??
I currently have three silver mollies, one yellow molly, one swordtail, one cherry barb, two danios and a small silver tail shark (lady at Petco said he would be compatible with others). The creamsicle seemed to have fin rot (the first fish I have had that had it) but the rest didn't seem to fit the description of any of the dieseases I seen in the diesease section of this website. Any reason these fish keep dying, have I missed the mark on something Or should I just stop buying fish at Petco? LOL I would appreciate any help at all.