Fish have many behavioral traits. Many species are aggressive towards other fish in varying degrees. There are also fish that are social and like to be kept in large groups. Before you choose your fish, you should find out how they interact with other fish. Fish from the cichlid family can be aggressive, some will tolerate many fish in the same aquarium, and others will kill any other fish you try to introduce in the aquarium. Fish of the same species also have different personalities. While one fish of the same species will not bother tank mates, another may terrorize. Many fish like to dig holes in the substrate and you may want to avoid them if you plan on keeping rooted plants.
Fish have a preference as to where in the water column they want to occupy. Guppies prefer the top 12 inches (30 cm), angelfish prefer the middle, and Corydoras catfish will occupy the bottom. Many species of cichlids prefer being near the bottom. Hatchetfish will occupy the surface of the aquarium.
Providing plenty of plants, rock, and wood will not only give your aquarium a natural look but it is also functional, giving fish a place to hide in case of excessive aggression.
Healthy and properly set up aquarium ecosystem often result in fish spawning on a regular basis. Understanding the spawning behavior of the fish you are keeping and setting up the aquarium to accommodate them will result in hours of enjoyment for the hobbyist.
Providing the proper rocks, sand, plants, wood for your fish will reward the aquarist with the natural behavior the fish exhibits in the wild. There are many books and website that describe fish reproduction behavior, a little research will pay off in the end.
Aquarium fish have life spans that range from less than a year to over 200 years. Killifish and many of the common livebearing fish (guppies, platies, swordtails, mollies) have short life spans, a year or two would be all you could expect. Guppies on average live about a year. So if you buy an adult guppy, don't be surprised if it only lives a few months in your aquarium. Many small fish have short lifespans, so if you would like fish that have a lifespan that last more than a couple of years, you should look into fish like larger cichlids, catfish, silver dollars, and goldfish.
Angelfish (a cichlid) live about 4 years on average, but it's not uncommon for them to live 8 or more years. The ram dwarf cichlid (Microgeophagus ramirezi) on average lives only a year, but it reaches reproductive age at only 2 ½ months. Many species of small tetras can live five or more years in the aquarium.
The eating habits of fish should be considered when setting up the aquarium and choosing fish. Many fish like to eat algae and plants, while others eat only live food. If you plan on keeping live plants, make sure the fish you introduce will not eat plants. Some fish like the clown pleco (Panaque maccus) enjoy wood in their diet, therefore you should consider keeping driftwood in the aquarium.
You should try to avoid feeding live food as much as possible, because live food can introduce harmful parasites, bacteria, and viruses to the aquarium.
Fish that are sand sifters like earth eater cichlids (Geophagus sp.), Corydoras sp. catfish, and loaches should be provided with fine sand as a substrate as it allow the fish to exhibit their natural feeding behavior. Discus are very slow eaters, and should not be kept with fish that are aggressive eaters that will out compete them for food. Discus will eat from the mid water column and off the bottom.
Mollies, mbuna cichilds from Lake Malawi, and Tropheus cichilds from Lake Tanganyika like to forage on algae all day long. Providing or promoting algae growth in the aquarium for fish that are adapted to eating algae can be beneficial.