Otto, or bristlenose pleco?

...why don't you just feed less?

Well, both of these are more algae eaters rather than bottom feeders. You want some bottom feeders, cories, a snail, or some large freshwater shrimp would be the best. All of these should have sand in their tanks, though.

Otocinclus are very sensitive to water parameter changes, and it may take a few to die before you actually get some that survive going into your tank. \

Bristlenose plecostomus is a little bit big for a 20 gallon, even without fish in it already.

A different option would be to get a small crayfish. They will attack and eat small fish, though. I've seen my blue crayfish eat a fish that's just as big as him. I'd get the smallest breed of crayfish you can find.

Some crayfish even eat fish's feces, keep your sand really clean.

Downside is that they eat plants.

I'd just get some cories, or maybe some apple snails. Although they can breed in freshwater, they only have one gender, unlike hermaphroditic malaysian trumpet snails and ramshorn snails. And if they even do breed, they lay their eggs above water, so they are easily removed.

But the easiest solution to your problem would be to just feed less.

Seriously, try to think of other options that don't include spending money on new fish.

left overs? as in food? if their is left over food feed less. you can easily over feed fish to begin with and cause death. also over feeding is bad for your water chemistry.

a bristlenose pleco needs 20gals at the LEAST i believe. and by 20gals i mean a 20gal space for itself so if you have a tank thats 20gals with other fish this will NOT due. many people think when they say Xgals minimum it means the tank itself needs to be xgals not taking into consideration that the fish itself needs that much space ALONE.

so if you got 20gals to play with go bristlenose if you want as for otto's idk anything about that. i myself would rather leave the tank bare more room for your other fish = better. people tend to over stock their tank because they think it's bare -.-" having a bare tank isn't a bad thing it's actually a good thing.

Neither. Both are mainly vegetarians that prefer to graze on algae. Feeding less and syphoning excess away is the best solution