Here are some important things to consider:
- Add your fish a few at a time.
It is important not to overwhelm your tank environment by adding a bunch of fish at once. Buy 2 or 3 fish and allow your tank to recover before you add more. - Goldfish are never a good idea.
Unless you are creating a goldfish tank, goldfish are not a great addition. Contrary to some popular theories, they are not even a good idea to use to cycle your tank. Goldfish are coldwater creatures and they will not do well at a tropical temperature. Goldfish also create a lot of waste. They can quickly overwhelm a new tank that is establishing itself. - Don't over load your tank.
An easy rule of thumb is that a freshwater aquarium will support one inch of fish for each gallon of water it contains. If you have a 20 gallon aquarium, it can support a maximum of 20 inches of fish. That means the adult fish. Fish grow. Don't buy 10 2" long fish and forget the fact that each of those fish are going to grow up to be 4" long. You have just seriously overloaded your aquarium. - Build a school.
If you decide you want fish that school, then build your school a few fish at a time. If you want a community tank, build one school and then begin to build another school of a different species. - Lonely fish turn aggressive.
If you select a fish that lives in a school and you don't provide it with friends, it will turn aggressive and nip other fish. Provide your fish friends of the same kind and they will, for the most part, leave their tankmates alone. - Large aggresive fish will eat their tankmates.
If you purchase say an Oscar and provide him with a tank of little tetras, you will eventually find a fat, happy Oscar and no tetras. Make sure you are not buying fish that eat their tankmates, unless you happen to enjoy watching fish being eaten. - Make sure the fish you buy live in the same climate.
You want to choose fish that thrive in the same temperature and pH levels. - Properly acclimate your fish.
Don't bring your fish home from the shop and dump them into the tank. Keep them in the bag and let them adjust to the tank temperature. Then slowly transition them by removing water from the bag and replacing it with tank water.