Each order of amphipods contains roughly 75 amphipods. Each jar will contain one or more reef-safe nylon sponges for the amphipods to burrow into. The jar is about 1/3 - 1/2 full of water as amphipods require a lot of oxygen.
Larger than copepods, amphipods are voracious detritovores - eating fish waste, leftover food, and some forms of n