Actually, Friendlies discovered some little creatures in their systems and at first were scared it would be yet another thing to need to get rid of but later they discovered that they were eating the detritus and keeping their net tanks clean.
Gammarus, also called scuds and side-swimmers, are amphipods. Most amphipods are marine, but there are 50 American species living in fresh water. They live in unpolluted lakes, ponds, streams, brooks, springs, and subterranean waters.
The body of Gammarus is laterally compressed and consists of a cephalothorax (head/thorax), seven free thoracic segments, a six-segmented abdomen, and a small tail (telson). They have seven pairs of thoracic legs, some of which are adapted for swimming and some for walking. This accounts for their Latin name, amphipod: amphi meaning "both," and pod meaning "feet or legs." Their eyes are well developed, and they have two pairs of antennae.
In general, Gammarus are much more active at night than during the day hours. They crawl and walk using their legs in addition to flexing their whole bodies. When Gammarus swim, they often roll over on their side or back (hence the name side-swimmer).
. Their environmental preference is for dark areas. They are scavengers, browsing on microscopic plants, animals, and decomposing material.
This is the genus of critters that the Friendlies found in their net tanks. I have them also and they are wonderful. The hang out sometimes right where the outflow of the tank is into the filter. The statesments above came directly from FossWebb a scholl science page. Cray fish can be used except they roam and will clog your pipes. I was at Olamana gardes one day when Glen found out the reason the pipe was clogged was a cray fish. There are midge fly larvae which are little red worms. I imagine others aldo and I hope people chime in and name a few of their helpfull critters. These are a part of a trully ecological system
I've heard of glass or ghost shrimp cleaning the detritus out of pond filters so well that the filters don't catch the fine solids well enough anymore for some people with ornamental ponds.
I know you posted this awhile ago...but have had some success with cherry shrimp in my 55 gallon indoor system keeping sludge from building on my pre filter sponges. They would still be working if the goldfish would stop eating them, lol. Without predators around though, these little guys will reproduce and keep eating. I also find that MTS and red ramshorn snails do an awesome job on them if there are no dead plant parts about.
If you have a lot of flow coming into the tank (read as water has a high O2 level) then you can also stock red worms in there...seriously. I have them living (underwater) in several of my in tank plant filters and in a pocket tower. They eat sludge, plants, poo & bacteria. They have no lungs and need to stay moist to breathe O2...so as long as there is good water flow, they thrive.