You could add snails.
Nerite snails cant sucessfully breed in fresh water, however that wont stop them laying eggs all over the place, lots of people find the eggs unsightly, but nerites do come in a variety of shell colours and shell shapes. The downside is most nerites really do prefer to be in brackish water at least for some time.
Mystery Snails and Apple Snails, are both a large snail which can go through a lot of algea, I have never had much luck with either of these snails because they tend to like water harder than my tanks. Both types I believe lay their eggs above the water so if you do want any more snails just remove the egg cases.
There are some nice Ramshorns, I have seen pink and purple ones, the down side of ramshorns is they will eat plants and they can breed into plagues, but if you have the pink or purple ones they maybe more desirable for selling to other hobbists. There is also a giant ramshorn which I think looks quite stunning with its swriling patterened shell. I have no experience with them, but they do look nice.
You could also get Japanese Trapdoor snails, and there are other trapdoor snails to which might suit your tank.
A word of warning with lots of shrimp from sheer numbers they will be putting a bit of a bioload on the tank, if you add snails which can be poo machines in thier own right it will increase the bioload.
