From my experience with both the PC and the iPhone versions, the medication for the diseases both say which disease they cure. The pink medicine (fungal treatment) mentions that it cures the fungus, which "looks like a green growth on a fish." The yellow medicine (ick treatment) mentions it cures ick, which "looks like white specks on your fish."

And from what I remember, if your fish has both diseases, you'd have to buy both of the medicines and cure them. When you purchase the second medication however, it will replace the first medication you bought so double-check that there aren't any fish in a different tank who have the same disease. Also, keep in mind that if a fish has been sick for a while, it may take more than one drop of medicine to cure them.

Hope this helps!