Your customers won't buy sick fish -- and they're less likely to buy fish from you if you have sick fish in your sale tank.

If the sick fish is valuable (worth more than $50, or hard to breed), buy the appropriate medicine (ick is the white stuff, green bubbles are fungus). If the fish isn't really valuable (worth very little, or you have a dozen of them), or if you can't afford it, your best option is to trash the fish . Sick fish don't heal on their own.

If your fish are losing health but don't have ick or fungus, then you need to research fish environment to allow them to survive. Vitamins from the supplies store will raise their health a bit, but they'll continue to lose health until they die.

Good luck!