I've ready many posts, guides, and tips on this page. I believe the "don't add it if you don't need it" philosophy. What I haven't figured out along those lines, since chlorine is cidal (kills) to algae, under what conditions would I need an algaecide other than chlorine? One point, somewhere I read that it can take a free chlorine level of 30 to kill some algae (ie a significant shock level). But having said that, why would I need copper, quat, etc? Thanks.