Diet - Most staple diets do a very good job of providing a broad range of essential nutrients required for proper fish health. However, providing a varied diet of food appropriate for your fish, in different forms, ensures better nutrition.
Stable water temperature - Proper water temperature that remains stable is one of the most important factors influencing fish health. Sudden temperature change (as little as 3°F over a 24 hour period) is extremely stressful and can compromise their immune system. This causes fish to be susceptible to parasitic infections such as Ich. Invest in a quality heater and thermometer.
Good water parameters - Start with a properly sized filter that has been cycled, see "Keep Aquarium Water Free of Toxic Compounds." Maintaining good water quality is also very important for healthy fish. A monthly partial water change of no more than 30% reduces nitrate buildup, removes debris the filter missed, and replenishes oxygen available to fish. Test kits provide an easy way to verify good water quality. Ammonia and nitrite should be zero, and nitrate in the safe range.
Observe daily - Regular observation is necessary to make sure everything is okay. Deceased fish or filter malfunction can cause aquarium water quality to become toxic very quickly, especially in smaller aquariums. It is very important to catch and remedy these problems in a timely manner.
Adding fish - Before purchasing fish, find out how large a species will grow. In general, the maximum stocking recommendation (maximum fish load) is one inch of adult fish per gallon of freshwater. When setting up a new aquarium, start with 25% of your aquarium's capacity. Do not add any more fish until ammonia and nitrite levels return to zero.
Research - The old saying "prevention is the best form of medicine" also applies to a successful aquarium. Understand the needs of the fish you select for your aquarium. When adding new fish, choose species with compatible temperaments. This will make fish keeping a fun and enjoyable experience.