Bettas have a reputation for pugnacity, to the point that another name they go by is the Siamese fighting fish. However, their aggression has very specific guidelines. A single male betta -- or a group of female bettas -- can comfortably share an aquarium with otocinclus cats.
Bettas can act very aggressively, but their aggression has many important limits. In general, only male bettas show serious aggression. Even then, male bettas usually fight only with other male bettas. The sole exception is when bettas mistake other long-finned fish like guppies for other male bettas. However, pet shop bettas have long fins from selective breeding. These long, flowing fins make them poor fighters compared with their wildtype ancestors, since it slows their movements and makes them more vulnerable to injuries.
Aquarium hobbyists usually call Otocinclus catfish "oto cats," or Otto Katz if they have a literary background. These small catfish are related to their relatives, the famous algae-eating plecos. Though oto cats also like to nibble at algae, they act like minnows or tetras in many ways. For instance, Oto cats school for protection in the wild and need to live in a group of at least six in an aquarium to feel safe. Oto cats need peaceful tank mates that will not harm them.
Ideal Tank Setup
Both species have similar aquarium requirements. Neither bettas or oto cats should be kept in bowls. Both species need a heater aquarium with filtration. Additionally, both species need densely vegetation areas of the tank to help the fish feel secure. An aquarium of at least 10 gallons can easily accommodate both a betta and a school of oto catfish.
Oto catfish are more sensitive to water chemistry than bettas. If you address the oto catfishes' aquarium preferences, a betta will be comfortable as well. Keep the water temperature between 70 and 79 degrees Fahrenheit, since both species come from the tropics. Oto cats prefer soft acidic water but can live comfortably in water with pH of 5.5 to 7.5, slightly into the acidic range. However, if you want to breed oto cats, you should keep the water in the acidic part of this range.