Copepods are definitely beneficial. They eat detritus and are a good food source for many fish and LPS corals. There are several types out there commercially sold, the most prevalent are Tigriopus californicus and Tisbe biminiensis. The Tigriopus californicus (Tigger Pods) are large and generally free-swimming, so they are eaten by more fish but must be replenished. Tisbe (ReefPods) are much smaller than the Tigriopus and generally live in the rocks, so you can seed your tank with them and they will populate it over time.
CJ