When you go to buy more fish, buy a bottle of Protozin. Open it and put two drops in the bag before the assistant ties it up. If you don't have a quarantine tank, it's the best thing you can do. Also, take as long as you can to acclimatise the new fish. Empty the bag (fish and water) into a bucket, then add a cupful of tank water every ten minutes or so. After about an hour of doing this, transfer them into the tank. I started doing this just over a year ago, and my new arrivals have never had any problems.