Inverts

The plant tank (inhabited by cherry red shrimp, pygmy corys and Otocinclus) has a homemade sponge filter - a piece of sponge over the intake of a HOB (hang-on-back) filter. Since the corys spend most of their time under dense vegetation at the back of the tank, there's a lot of essentially predator free space for smaller inverts. The result of this is a steadily growing population of small organisms - probably cladocerans. While I'd have no problem harvesting them to feed to the fish in other tanks, I am concerned about shrimp larvae. I have no good sense of how small freshly emerged shrimp larvae are likely to be. I wouldn't want to end up turning them into fish food (or worse yet, simply discarding them after I clean the sponge filter.

Cleaning the sponge produces a huge crowd of these guys - and a layer of "dirt" (aka POM - particulate organic matter). I need to find some way to separate the inverts from this stuff. Right now I have them in a small cup floating in the tank. While this allows the dirt to settle out, it doesn't help much because the inverts prefer to feed in the POM. Is there some way to sink the cup but not disturb its contents to much?