Choosing Compatible Tankmates For Your Aquarium Fish
So, you’ve decided to go in for a community tank [Read Species Aquarium Or Community Aquarium?], and now you’re wondering which are all the different fishes you could get for your aquarium. It does take a bit of research, but its not too difficult.
Temperament: For one, you need to check out the social skill or the temperament of the fish. Some aggressive fish like 
chiclids do very well together, but keep them with peaceful fish, and you’ll have a full scale slaughter on your hands. Keep Tiger Barbs, with a betta for instance, and there is no way the betta is going to live to talk about it. Sometimes, an aggressive fish will keep more peaceful tank mates away from food or ideal hiding places, causing the friendlier fish to die from malnutrition or become more disease prone due to stress. Fish like the neon tetra are so timid, that while they are generally considered suitable for the community tank, my experience favors keeping them in a species tank. I once kept these beautiful fish that the aquarium shop kept calling the golden pinch, which quite literally stalked the neon tetras in the tank. As in everything else, beauty goes more than skin deep for fish as well. Another moral, is that the guy at the fish shop may not know as much as you hope he would, so do your own research.
Size … does matter. As a rule of thumb, most fish will eat anything that they think they can get in their mouth (that it may be their own fry carries little merit). Try and pick fish that are approximately the same size.
Water Temperature: Different aquarium fish require different water temperatures to thrive. You should make sure that the temperature range in the tank is suitable for the fish that you select. Most the aquarium fish will survive in the same temperature range, but it is better to make sure.
Water Parameters: Once again, different fish have different needs when it comes to hardness, pH, quality of water, saltwater content, etc. The degree to which different fish are able to tolerate less than ideal conditions varies, so this is another factor that you need to consider.
Tank Setup: You need to remember that your aquarium is pretty much the world for all your pets, so they need an environment which mimics their natural habitat as much as possible. Some fish are shy, and need caves or other hiding places, some need a soft substrate, some like the Congo Tetra need a dark substrate, while others may need a planted tank. In short, make sure there is something for all of them, or that all the fish in your tank have the same environmental needs.
Pic via here.
