As you are stocking your new tank, you are going to want to put fish together that will be good tank mates. There are certain fish that do really well together, and there are some that you should absolutely avoid placing together.
First, use common sense when choosing your fish. Do not put coldwater fish, such as goldfish, with tropical fish, like tetras. Think about the size of your fish. If you have small fish, a large fish will think they make great dinner. Finally, slow, peaceful fish will be antagonized by fast moving, aggressive fish. Avoid putting fish together that are in the same family if they have similar body shapes, as they will most likely not get along well. If you do want to add more aggressive fish, add them last, and make sure they are smaller thank their tank mates.
Here are some common tank fish and the fish they should not be housed with. If you want to raise Malawian Cichlids, do not put them with any other fish. They are lovely, but very aggressive. You can put them with Tanganyika Cichlids and barbs with caution, if you have a large tank. Cichlids may work out with bottom feeders, but again, watch them closely, and have a plan if they do not work out.
Angelfish have large, delicate fins, so do not put them with any fin nippers. Barbs, Cichlids, and Discus are probably not the best tank mates for angels. The same rule goes for Bettas. Also, do not put male Bettas in tanks with other male Bettas. They will attack each other.
Cory Cats do well with most fish, but use caution when putting them with large bottom feeders, as they may become dinner. Use caution when housing Discus, as they are very peaceful fish and will be aggravated by active fish. Species to avoid pairing with Discus are Gouramis, Plecostomus, and sharks.
Because Gouramis can get quite large, avoid putting them with small species, like guppies and tetras that may become lunch. Barbs are fin nippers, so use caution when placing them with any fish that have long, flowing fins. Killifish also tend to like to snack on fins, so the same caution is needed.
Live bearing fish are usually compatible with ach other. Mollies, Guppies, Swordtails, and Platies all do well together. Do not place live bearing fish with Cichlids, and use caution when putting them with large cats, as they might become lunch.
Choosing compatible fish is not hard, if you use a little common sense. Pay attention to the water requirements of your fish. Some fish need soft, slightly acidic water, while others prefer hard, alkaline water. Water requirements will be a good indication of the compatibility of the fish. If in doubt, ask you fish store associate. They should know which fish make good tank mates. If they don’t find a new store! You need to be buying fish from people who know what they are doing. Having tank mates that get along will ensure your fish are happy and healthy for as long as possible.