Guppies and tetras have a lot going for them. They are colorful, inexpensive and get along with most other fish. Additionally, there are many species of tetra and many varieties of guppies out there, with a huge range of colors. Take care when selecting tank mates for them, but there are many options available.
Schooling
Schooling bannerfish (Heniochus diphreutes) image by stephan kerkhofs from Fotolia.com
First and foremost, both guppies and tetras should be kept in a group of the same species. As schooling fish, they generally feel most comfortable with others of their own kind. Both guppies and tetras are adapted to living in large groups in the wild as a means of protection. In order for them to feel safe in an aquarium, keep them in groups of at least three to six individuals of their own species.
Small cichlids
aquarium image by Henryk Olszewski from Fotolia.com
The cichlid family is a large family of fish. Some of them are known for being aggressive. However, smaller members of the family are more easy-going and ideal tankmates for guppies and tetras. Rams are one type of cichlid that goes well with tetras. They are small, colorful and require peaceful tankmates. Rams can come in several colors, including blue and gold. The krib is similar and also comes in several colors and has a peaceful demeanor.
Plecos and Ottos
Plecos and ottos (short for Plecocostumus and Otocinclus) are other ideal tankmates for guppies and tetras. These fish have a reputation for eating algae and help clean a tank, though they don't eliminate the need for maintenance. Many different species are available, with many different colorations. A smaller species of pleco is probably best suited for an aquarium with tetras and guppies. Most smaller plecos and ottos are peaceful, but larger plecos can be aggressive and territorial.
Cory Catfish
Corydoras catfish, cories for short, are another great match for tetras and guppies. Cory catfish come from the Amazon drainage, where most tetras come from. They are bottom-dwelling (helping to balance a tank with tetras), but they also school and act more sociable than most tetras. There are dozens, if not hundreds of different species of Corydoras catfish available in a wide variety of colors and patterns. All are peaceful and will not harm tetras or guppies.
What to Avoid
Aquarium fish-Capoeta tetrazona image by SPLAV from Fotolia.com
Guppies and tetras usually mind their tankmates and are rarely a problem. However, other fish may harm them. Some larger fish, like the freshwater angelfish will eat guppies and tetras. Other fish, like tiger barbs, may attack guppies' flowing fins and nip at them. Oddly, aggression in tiger barbs can be mitigated by keeping them in a large school. In short, tankmates should be peaceful and not large enough to eat the guppies and tetras.