I second Beth's advice to have a quarantine tank. It isn't a question of whether a fish will bring in a disease that will wipe out your tank, but when that will happen. A simple, inexpensive quarantine set-up will 100% prevent that. The tank can be as small as a 10 gallon tank with no sand or rock, just a piece of PVC pipe for shelter, a heater and a bubble-up filter in the corner. The filter material should be kept in a mesh bag in your display tank so that it gets cycled when the display is cycled, and will be loaded with bacteria. Then, when you get a new organism prep the quarantine by putting the filter material into the bubble-up filter and, voila, the quarantine tank is cycled and ready for the new addition. Isolate for at least 6 weeks (this is hard because you really want the new addition in your display, I know) and be ready to treat for a variety of diseases while you acclimate the new arrival to aquarium food.