Otos are very sensitive algae eating catfish--many are wild caught and are therefore unwilling to accept most prepared foods. Due to their sensitivity and their very particular eating habits, they only belong in stable long-established tanks that have a constant supply of algae. And it is not only about having some algae, it is about having the correct types--of the algae that can occur in the aquarium, otos are particular about which types they will eat.
Because of their algae needs, I also believe the bigger the tank the better--larger tanks have much larger potential to produce algae. Also, otos are a social species that should be kept in groups of at least three.
As for where to get them, that can be tricky. Many otos are starving before you even bring them home from the pet store due to lack of algae.