MK, I personally don't think a BN would work in a 20gH. A 5" fish with a relatively high bio-load 24" long tank doesn't bear thinking about, IMHO. Mine was bad enough in my 29gal when he was 3"...now that he's bigger, I wouldn't have him in anything less than a 40gal breeder.
That being said, if you can get one that is strong enough, a rubberlip pleco might be a good choice. They only get to be about 3.5-4" and have a significantly lower bio-load, from what I've read. Or nerite snails always work.
Otos aren't recommended for new/young tanks, as they are all wild-collected and sensitive to water parameters. They also don't take readily to prepared foods, like algae wafers, or even veggies (I know some people have gotten theirs to eat zucchini and cucumber, but mine never have), so you have to have a readily-available, constant source of algae for them to eat..