Scuds (Gammarus sp.)
Scuds (Gammarus). These side-swimming amphipod crustaceans, Gammarus pulex and its gammarid cousins, are mostly too fast and too strong-shelled and too big for my apartment-sized fishes. Hyalella azteca, the "little glassy transparent one from Mexico" as its name describes it, is the species I ought to have tried. You can raise them in your fish-free plant nursery, where they'll scoot about devouring decaying leaves and other organic detritus and grazing the biofilm, but each time you use it as a quarantine tank for small fish, you'll lose a generation of the smaller gammarids. In the aquarium, uneaten adult gammarids will survive indefinitely, their smallest progeny offering snacks. Besides shredding organic detritus gammarids will eat smaller critters, like daphnia and copepods. They'd eat infertile fish eggs, certainly, but I'm pretty sure they'd eat live ones too.
Links. A simple amateur's introduction to Hyalella azteca. A biologist's encyclopedia entry on H. azteca. Hyalella azteca at Wikipedia.
Culturing scuds as a live food is the subject of a 2008 post at the North American Native Fishes website. Colin McCourt's "Pardon my Gammarus" post at the British Killifish Association board offers plenty of detail about the life cycle and culturing of the Mexican gammarid Hyalella azteca.