Hi,
Yes, I mean Asellus aquaticus. You have to catch the animals and only them, or to have them by a live food supplier, because you must care not to bring dangerous planaria (Polycoelis) or hydra with the stones, the wood or the leaves, that are deadly for eggs and/or jungfishes.
There is a good live food breeder in elsass (http://www.aqualiment.com/v2/), but the web pages are in french, and I don’t know if they send parcels to germany.
There is another freshwater invertebrate species, “shrimp”, which could live in our tanks, the gammarus, but it can bring fish diseases and I never succeed to let him live long or to breed in warm water tanks, and it’s not so good a cleaner as the asellus. Asellus are not very good algae browser, but “work” the dead leaves and so on, so that it could be he decompose by other cleaners like snails and they clean rest of food too.
Hugues