What was the disease? Unless it was some fish form of Ebola or something, you should be able to clean the tank and re-use it. For most infectious outbreaks, you can disinfect with vinegar or ammonia (but not both), do a soak & scrub, and rinse REALLY well. When you think you've rinsed enough, rinse 2-3 more times. DO NOT USE SOAP. There are certain forms of fungus that can only be eradicated with bleach, but I've never dealt with them and am not well informed enough to give an opinion on that.