Guppies are a type of community fish, fish that get along well well with others. There are many choices of companion fish for the guppy, including a number of catfish. Many catfish are also community fish, with similarities to guppies.
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The plecotomus family is a big family of catfish from South America, particularly the Amazon basin. Plecos are famous as algae-eaters, though you can't expect them to do all the aquarium cleaning for you. Some species like the bristlenose pleco stay relatively small, around 4 inches long. Others can get much too large for most aquariums at more than a foot long, so select plecos for your guppy tank carefully. They will require feedings of vegetables matter. Sinking veggie pellets (available from pet stores) are a good way to make sure they get fed.
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Otocinclus are very small catfish related to plecos. They have many things in common with guppies and get along well with them. "Otto cats" do their best in schools of at least six. Like their larger cousins the plecos, they benefit from vegetable matter in their diet. They look like very tiny plecos. They also help keep aquarium glass free of algae. Several varieties are available, though the most common is a black fish with a white underbelly.
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Cory catfish, short for the Corydoras genus, are another community catfish that get along great with guppies. They like to school near the bottom of the tank and help eat leftover food, preventing it from fouling the water. It is a huge genus of fish, with many different colors and patterns. Cories complement guppies well, as guppies tend to school in the upper part of a tank while cories tend to school at the bottom of the tank.
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The upside-down catfish is the only fish on this list from Africa. They do swim upside down. Like the cory catfish, they are happiest in groups of at least six. However, they are slightly larger than most cories at around 4 inches, taking up more room in your aquarium. They require some vegetable matter in their diet, as well as meaty foods like bloodworm, brine shrimp and tubifex worms. Additionally, they like to hang out under pieces of driftwood.