In fact, beyond "if it ain't broke, don't fix it" no medicine should be used unless the disease is confirmed to be what you are treating. That is, no preventative medications ever.
The reason is simple, if you keep dosing preventative medications, all you do is breed medication resistant strains of the bacteria/parasite. Then, when you need the medicine for a real outbreak, the strains are completely impervious to that medicine. Worse yet, your home-grown strain of certain medicine resistant bugs get out (you trade fish at a show, sell some extra stock to the LFS, or just give them to friends) and now the strain gets out.
If you think it cannot happen, just look at the problem staph infection have become in hospitals. It used to be several years ago a staph infection was nothing, penicillian knocked it out cold. But nowadays there are several resistant strains, and even some apparantly resistant to all the known medicines. The number of staph deaths rise each year. The rate of mutation of these creatures, and thereby the rate they acquire immunities far outpaces the rate of our discovery of new medicines. It is for reasons such as this that many European countries have banned over the counter sales of antibacterial agents -- something I believe the US could learn from.
