If you look closely enough your tank is probably crawling with microorganisms. If you really want something in your tank, some detrivore... first I can think of is maybe live tubifex worms. Alot depends on what kinda fish you have. Generally anything you add can also pass as fish food. You may find that some organisms can not become established.

 

Although you might attempt to culture worms in your main tank, it would require less than ideal (substrate) conditions and you might be further ahead culturing live foods in a separate tank and just collecting for feeding in your main tank. Culturing worms in your main tank would require a somewhat 'nasty' substrate to support them, which may present other negatives and not produce the desired result.
On the other hand, if you have a well planted tank and don't use a gravel siphon to keep the substrate clean, dropping in some tubeflex or other worms in the cover of darkness may not hurt - ???

(just my $.02 worth)

 

dunno but I have usually have planiaria, nematodes, and the occasional rare tubifex in the substrate of my 55 gallon. You don't normally see them unless you suck up some gravel in the bucket or are cleaning a filter. Also the surface organisms if the plants grow in REALLY thick and form a mat on the surface. The offer protection for rotifers, water fleas and stuff.

 

Malaysian Livebearing Snails or Malaysian Trumpet Snails (MTS) will live in the substrate and provide the burrowing your looking for.