As above, no animal can clean the tank as a water change would do.
However, you could easily have shrimp and snails.
I would go for a few Amano shrimp (they are bigger and come out more often) and a couple of apple snails (buy nice colours off the internet).
A couple of nerite snails would be good, but beware that they can slowly starve is there isnt enough algae, so only get one or two.
Also, don't bother with snails if you have acidic water, as their shells will eventually corrode. pH 6.8 isnt much a problem as long as the snails are fed enough food that has the right minerals for inverts (like shrimp food) or if you dose some extra calcium into the water, but pH 6 would be deadly in the long run.