If there's a two word solution to all the worlds fish keeping problems I would say WATER QUALITY. Two major ways to maintain water quality, filtration and water changes. A few word about filtration, you can't have enough. If your into fish you'll want to make the investment in a good filter. Don't buy a filter for a twenty gallon tank that says "up to twenty gallons". This will only lead to frequent filter media changes (filter media isn't cheap) and perhaps dirty water. If your only running one filter, remember, the bateria that grow in that filter element are just as important as the filter itself. When you throw your filter elements away you throw your bacteria away. We buy filters rated for about twice the capacity of the tank keeping in mind if we ever want to get larger tanks the filters can be reused. It's also nice not to have to change the filter media every two weeks! We have some personal favorite filters, Aquaclear makes an excelent filter in a range of sizes from the Aquaclear Mini to the Aquaclear 500. These filter have a sponge media that's good for bacteria growth and can be cleaned easily by rinsing it off (use tank water to rinse so you don't kill the bacteria) and used again.
As for water changes, we do about 30% water change once a month. Many people recommend "aging" the water, but if the PH of your tap water is very close to that of the tank, the tempertures are the same and your town doesn't have high chlorine content in the water you can try water from the tap (we always do!). Be careful!