Oto's can be difficult, I keep 2 in our DP tank, I'm not teaching you to suck eggs, but in the store, look out for Oto's with nice rounded bellies and get at least 3 as they don't do well on their own, honestly they do a magnificent job in our tank and the puffs do not go ner them.
Also, I keep 3 red claw crabs in there who eat what the DP's miss (we still do a vacuum every other day) the crabs really do best in brackish but we give them a brackish bath every week and they have wood high enough to let them get out of the water to "breathe". Give the Oto's another go.
In my experience, all the larger snails eventually eat your plants !!