Eels are a large family, but mainly the subfamily, Garden eel (Heterocongrinae) are relevant in relation to aquaria.
Garden eels live together in groups. They require enough sand in the aquarium substrate to make their small but deep holes, also the more one wishes to keep the larger the sand surface needs to be.
The minimum number kept, should be 3, the optimum is between 6 and 10. Each eel however, needs at least an area of 12 by 12 inch (30 by 30 cm).
They eat zooplankton from the water column, which is difficult to maintain in an aquarium. At first one needs to have live Artemia and preferably a healthy population of zooplankton. Later on they can be fed with frozen foods, provided this can be fed in such a manner that it floats gently past the eels so it can be caught.