The Almighty Amano shrimp! This little bugger adds very little to the bioload and will have your tank spotless in no time. Just make sure your water parameters are roughly in line with the recommendations. I don't recommend crayfish because, well, if you want to annihilate everything in a planted tank within a week, add a crayfish. Crabs, or at least the RCCs and vampires I've kept, ate zero algae. HTH!