Common water conditioners people add are dechlorinators, as what Blue mentioned. Most water suppliers add chlorine/chloramine to their water supply to kill bacteria. Chlorine/chloramine is harmful to fish. Chlorine can easily be removed if the water is aged, by means of storing water in a container and using a powerhead/pump to agitate the water. Chlorine will then disipitate. Unfortunately, the only way to remove chloramine is using a dechlorinator, which bonds with the chlorine molecules. The unfortunate by product is ammonia (chlorine + ammonia = chloramine). There are other products, such as Seachem Prime, that also bonds with the ammonia. Some dechlorinators also help in aiding the slime coat on fish.
Regarding the fish, visit the LFS (local fish store) or LPS (local pet stores) and look at their fish and see what appeals to you. List down the names of the fish and then do some research as far as compability and habitat. Often, the info on the fish at these stores are incorrect regarding max size and compability.