A major factor in determining how successful your aquarium will be is the compatibility of the fish you put into it. One wrong choice of fish can lead to disaster and the destruction of other inhabitants.
Some fish are shy, others more boisterous and continually on the move and some just seem intent on murdering everything else in the tank. Communities can be made up of fish of different temperaments so long as there are plenty of hiding places for the shy species.
Boisterous species should be kept in a group or shoal so that their attentions are focused on each other rather than on the shy inhabitants. The more aggressive species should be avoided completely in a community set up. As most of us probably aware, big fish tend to eat small fish as a part of their natural diet so try to aim for a community of similar sized fish. Remember, this is the full adult size of the fish and not the size at which you buy it. It would not be a good idea to put a 2" Oscar in with 2" Swordtails although I've seen someone try it.
Ideally all fish should be kept in conditions that are as close to their natural habitat as possible. However, species which naturally live in soft acidic water, can often adapt to harder, more alkaline water and vice versa, providing the change is not too sudden. Most commonly available community fish are of this more adaptable type and if purchased locally will probably be used to the local supply. If you are in any doubt, ask the retailer if he is keeping them in special water conditions.
There are several hundred species that are suitable for the community aquarium and many good books available on the subject to help you decide what to keep. Once you have decided what you want, have a close look at all the fish in the tank. Choose those which have their fins spread and are actively swimming around the tank. Don't be tempted to buy fish that are showing any signs of ill health such as clamped fins, or shimmying (a slow side to side motion whilst not moving through the water). Look for fish with a robust body shape, thin or wasted fish should be avoided as these may be seriously ill.
Once you have made your selections, make sure that the retailer catches the particular fish you want. Introducing your new fish. Be careful and take the right precautions before introducing new fish and they should soon settle in. All new fish should be quarantined in an isolation tank wherever possible for at least two weeks before you introduce them to your community tank.
The isolation tank need not be anything special. A bare tank except for a small box or foam filter, a heater and maybe a few rocks for cover. Check the fish every day to make sure they are looking healthy and happy.
All equipment such as nets, should be used for this tank only and it is a good idea to wash your hands after working in it.
When introducing new fish into the aquarium from the quarantine tank, turn off the tank lights and feed the fish in the main tank. Open the bag and float it at the surface for about 20 minutes to allow the temperatures to equalise. Check the pH of both your tank and the water in the bag to make sure they are similar. Add tank water to the bag over the 20 minutes so the fish gradually adjusts to the new conditions. Keep a close eye on the fish for stress while they are in the bag. Some species are more susceptible to stress and may have to be released much sooner.