One of my favorite resources contains a section devoted to Otos.bettagurl said:an otto would be very unhappy,they should be kept in groups,I wouldnt keep one(alone)in less than a five gal....
For this reason, I wouldn't keep an Oto alone, regardless of the size of the tank.Here's the thing: no vertebrate vegetarian can digest cellulose, not one! so each carries a species-specific community of anaerobic bacteria (and some protozoans) that do the work.
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... Otos share gut bacteria: their need for the company of other Otos isn't merely social.