Saltwater fish, or marine fish, tend to be substantially more colorful and eye-catching than freshwater fish, but saltwater tanks need to be precisely controlled or disaster could strike. Because of this, freshwater aquariums are considered to be the easiest for beginner enthusiasts. In both cases, you have to choose fish that can co-exist peacefully without fighting.
If you're looking to populate your tank with your own versions of Disney's Finding Nemo, you'll need a small saltwater tank of about 20 gallons. The little clownfish is very cute and popular, and reaches a maximum size of 3 inches long. Most clownfish are bright orange with white and black stripes, like Nemo, but some species are just black and white. Also known as anemonefish, these easy-going guys do very well in aquariums with other types of fish as well as groups of their own species.
Tangs, also known as surgeonfish or doctorfish, are popular choices for large seawater aquariums. They have flat bodies and sharp spines beside their tails. Don't keep more than one tang species in the same aquarium since they have a tendency to fight -- unless you introduce multiple species of different sizes to the tank at the same time so they're not battling over who had the territory first. However, they do get along with other species, like clownfish, as long as they have enough space. This fish grows up to 12 inches long and needs a tank that's at least 180 gallons for one mature tang.
Saltwater or Freshwater Angelfish
Angelfish are available in both freshwater and saltwater species, with the saltwater varieties being the most vibrantly colored. For your large saltwater tank, choose a large marine angelfish who grows to be 8 to 12 inches long and needs a 125-gallon to 180-gallon aquarium. A dwarf angelfish reaches a maximum of 4 inches long and is happy in a smaller saltwater aquarium of 55 gallons. Only keep one marine angelfish in a tank -- fights will break out between angelfish of the same species, but they get along well with most other types of fish, including clownfish and tangs.
Freshwater angelfish grow to a maximum of 6 inches long and come in many color variations. These fish get along well with their own kind as well as others, such as goldfish and tetras, and can be kept in a small freshwater aquarium of 30 gallons.
Goldfish are some of the best fish for beginner aquarium enthusiasts, and despite the name come in many different colors including black and white. Forget what you may have learned as a child -- you can't keep a goldfish in a bowl. These fish require at least a 30-gallon aquarium.
Members of the carp family, the sizes and features vary dramatically, with some fancy species having long tails or bubble-shaped heads. Starting at 6 inches long, some varieties of goldfish can reach a length of 10 inches or more.
Freshwater Neon Tetra
The tiny neon tetra is one of the most common freshwater aquarium fish. Sporting a shimmering display of red, silver, green and blue with a transparent fin and tail, this little guy is pretty striking. He reaches a maximum size of 1 1/2 inches -- 2 inches for the "jumbo" variety -- and has a long body and blunt face. He can live up to 10 years unless his environment is allowed to deteriorate to the incorrect pH level and temperature. Since he is not aggressive he can live with other peace-loving fish like goldfish. These fish require at least a 10-gallon tank.
Freshwater Betta Fish
Betta fish are hands-down one of the most popular and colorful freshwater fish available -- and also the least likely to get along with others. Also known as Siamese fighting fish, you'll often see the males housed in their own little containers in pet stores. This fish grows to 3 inches long and looks delicate with his long fins and tail, but is actually quite hardy. Only keep one male betta in the same tank; the smaller females can live together, and any betta can live with a gentle species of fish who won't nip at his fins -- goldfish are a safe bet, but not tetras. These fish can live in a 1-gallon tank or larger.