Imo a catfish Is always a good addition, and they have the bonus of eating a lot of food that doesn't get eaten right away, but you should try feeding the birchir little less. However if you are going for a catfish a way to choose is google freshwater aquarium catfish then have a look and see what catches your eye, then do sum research on that fish, so is it comparable with the birchir, will it grow to a good size, does it have any special needs, etc...
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