Columbian sharks are often seen in the trade but for unknown reasons by many experienced aquarists. A columbian shark can get over 2 feet long, and they are schooling fish, meaning they need atleast 2 other companions. Columbians are also often sold as freshwater fish, while they are actually brackish and will not survive the long healthy lives they could in a tank of a proper size, with some of its own kind and water type. Please, for the sake of the fish, bring him back.
Plecos (sucker fish) get very large as well. They can get over 18" and in case you haven't noticed already they are very messy fish, meaning they poo alot! The need large tanks usually best over 55, with feedings of zuccini and cucmber to keep them fat and healthy. Please, take this fish back as well, so someone of a proper tank size can keep him.
Clown loaches are another problem. Apart from them liking to school, as already mentioned, they also get very large. Around 10-12" if not bigger. While they are beautiful fish, and full of life when healthy, being in that small of a tank, will surley do them no good. They also need a tank with a minimum of 55 gallons. Please, bring him back as well. They are very pretty fish, but just won't suit your tank at all.
Ok, now that I've gone over what can't survive, will start with what can. Guppies, a great beginner choice. However, as froggy mentioned above, they should be a 2-1 or a 3-1 reation females-males because the males will continusly pester the females to mate. Also, very quickly you will have lots of little guppies in you tank. That isn't nessicarally a bad thing but can be a bit of a pain, especially if you choose to try and raise them all to adulthood. Otherfish you oculd keep comfortably in your aquarium, would be swordtails, platies, cory catfish, tetras, and rasboras. Please, rethink your total aquarium plan, and try again!
