Here's another vote for the Oto's. While best kept in sizes 6 and greater, I have had great luck in my 20. I started off with 3, 1 died (they apparently have a HIGH mortality rate), then 2 weeks ago I went back to the LFS
and got 2 more (both have been great).
You could easily get 4 in your tank and not have a problem with overstocking, as they are small and stay small. While they do produce a good amount of waste as any algae eater does, due to their size its virtually insignificant, and their benefits outweigh the negatives.
They are also very interesting to watch school. When I had the 3 I didn't see too much social behavior (probably because one was ill), but after adding the last 2, I normally find 3 of them together roaming the tank, and one of the originals is more solitary.
If buying from an LFS
there are a couple things that are very important to get survivors:
1. Buy the biggest ones they have, these are generally heartier fish and will survive better during acclimation
2. Acclimate very slowly. Float the bag, add small amounts of water from the tank to the bag, then add the Oto's.
3. Find out when they get Oto shipments in and wait a couple days after the shipment arrives. Yes you might miss out on some of the larger ones that get bought right away, but you might have also avoided deaths.
4. Make sure their diet is supplimented with raw produce (cucumber, squash), algae wafers, and shrimp pellets (surprisingly my Oto's love the shrimp pellets?). While it may appear they have all the food they need, there might be a type of algae they are not likely to eat, and during the acclimation period you want them as strong as possible. They should always have FAT chubby belly's and be pooping almost constantly.
much more enjoyable to watch then snails!