technically.....yes, some fish can. should you submitt them to it? most people on these boards say no. most pet stores will tell you that it's ok to use damsels to cycle your tank, i've even heard of people using clowns since they're related, but the fish friendly way to cycle a tank is with a bunch of uncured live rock or some shrimp from the grocery store stuffed into some pantyhose. damsels are liked by few because of their aggressiveness and the fact that they are territorial and hard to get back out of your tank if you have any rockwork. also, a safe guesstimate would be that up to half of however many you get will die during the course of your cycle from stress.