Personally, I think the 3 gallon is fine, make sure to buy an appropriate sized heater that will not overheat. Bettas need it around 80. I had to put a samll apple snail in with mine and he was just fine with it, although gave it dirty looks at first. Make sure you are getting Pomacea Bridgesii, which is the smaller one that won't eat plants. I would not recommend the Otos because they need a lot of algae and only eat certain kinds, they are hard to supplement because they often won't eat algae wafers.