I have a second hand tank. It was cleaned and cycled before any fish were added. The first three fish in the tank were a black and red gold fish, a cold water guppy and an algie eater. The guppy died first within four days. The gold fish died two days later. The guppy seemed healthy until I found him dead. The gold fish began swimming oddly on day five. On day six He seemed bloated and swam on his side. There was no external damage or signs of illness. I assumed the guppy had starved because the gold fish was such a pig. The gold fish appeared to have over eaten, and died on day six. The algie eater seemed fine. I watched for 2 days and the algie eater still seemed fine. I changed the water, and put in a betta. My poor betta died within 24 hours. He had white cotton like fluff on his body. It began at the tops of his fins and spread very rapidly. I believe this was fungus. The algie eater started to show signs of illness at this point too. Lose of colour and a small amount of fluff on him too. I treated the tank for fungus right away. The algie eater seems to be doing better now. (Hard to tell) I've learned that fungus is a secondary infection. How do I determine what caused the initial problem? I want another crowntail betta but I'm afraid to put any more fish in the tank until I have the problem solved. Please help!