Is either one more or less beneficial. Seems the amphipods are out numbering the copepods in my fuge 100 to 1. Are the copepods being eaten by the other? Also are amphipods bad for a reef tank?
Both are good and fill a different niche in a reef tank.
As you probably already know, amphipods are much larger than copepods, and are therefore much easier to spot. Copepods generally avoid clean glass, so let the glass get a thin layer of algae and I'm guessing you'll see a lot more copepods. Then imagine that kind of coverage on the rock, substrate, and other surfaces in your tank.
Over time you'll probably see fluctuations in these populations based on tank conditions.
Aren't the little organisms living in our reefs fascinating? Sometimes I find as much joy watching the refugium as I do the DT.