The Quarantine Tank
Should you notice one of your fish has been injured, or seems ill, remove it immediately to a quarantine tank with the same temperature water to keep an eye on it. This can be a small tank, but it's an important one. If the fish is sick, you can tend to it in the quarantine tank. You may also prevent all your other fish from getting sick.
If you have live plants in your tank, make sure you take off brown or dead leaves before they fall to the tank floor.
Keep a record of when you add fish to your tank. Never add more than a couple at a time and, ideally, keep them in the quarantine tank for a week before you put them in your main tank, for their benefit and for the benefit of your other fish.
Aquarium shelves are full of treatments, from chemicals that kill algae to chemicals that control diseases. Many of these can have unwanted side effects. The best medicine is maintaining good tank conditions.