Just a follow up to the last post.When ever you do detect disease in your fish prior to cleaning your aquarium I have always set up another tank where I transfer the fish so I can treat them with certain medicines depending on their type of illness. I'm not an advocate of using medicines in a fish tank. This is absolutely the last resort to cure what ails the fish. The best medicine for aquarium fish is to keep the water at the right temperature,PH levels within the species range,ammonia and nitrate levels to a minimum and frequent water changes. The most common disease aquarium fish get is what is called ich. The fish will develop small white spots all over and if not treated right away the fish will die. This disease is is always present in your aquarium to some extent but it can be kept at low levels if you do the things I mentioned earlier. Getting back to cleaning the tank ,when you suspect disease in it I make it a point to either throw away the old gravel or boil the gravel that was in the tank to ensure the gravel is sterile. More or less sterilize everything in the tank. When you set the tank backup keep in mind when you sterilize everything you are going to need a longer tank cycling time to get the beneficial bacteria for the fish built up. One other thing I will do after having a major overhaul to a tank is to start putting maybe one or two inexpensive fish in it like guppies to see how they react to the water. There is a ton of information I have on this one subject so if you have any questions please feel free to ask.