Copepods

Several months ago I had a refugium that was loaded with copepods. The population kept expanding. Then, all of a sudden, probably about 4 months ago, most of the pods were gone. The only things in my refugium are brittle stars, astrea snails and 1 hermit crab. How could they disappear so quickly?
Well lately I've noticed some in the corners of teh fuge again. Over the last 3 weeks, they seem to be increasing again. Anyone have any explanation? My water parameters have been stable the whole time with practically zero algae.
SG 1.026, nitrate 0, phos 0, calc 380, alk 7, temp 79.

The fuge is only 4 gal but there were a few thousand pods in there. There isn't any access to the fuge from the display tank so they couldn't have been eaten. They literally disappeared in under a week yet there were still amphipods swimming around. The brittle stars in the fuge are micro brittles so it couldn't be them either.
Just one of those weird things.