I really wondered about those kinds of things when I first started playing Fish Tycoon (a million years ago)!
Most people simply do what Buttoneer suggested:

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The Ick treatments will also just work if you blast the tank with the stuff. the instructuions talk about 'every half an hour' but I find no such delay necessary.

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I never wait at all before administering repeated doses, since I usually run the game at fast speed, and sick fish continue to lose health until they're cured. There is some degree of finesse possible in treating fish, if your 'aim' is good, that will (in general) require fewer doses of medicine, but it can get frustrating finding exactly the right spot to drop the medicine to effect a first-time cure. Precision-oriented people who like a challenge can go for the 'medicine sweet spot' to see the fungus or ick cured on the first try. It requires precise positioning, and there can be nothing in front of the fishie to be treated, or you'll fail in your attempt.

Dumping medicine of either kind into a tank of healthy fish appears to do no harm, but it's probably not preventative, either.