When setting up a tank and tap water was just used. No fish are added for 2 weeks. Would a dechlorinator still be needed? Second part to the question, if the answer is yes, if there's fish already in the tank is it safe to use the dechlorinator? Thanks in advance, just asking since I asked my dad if he put dechlorinator when he set up his tank a month ago and he said no. Some of his fish died when he added them and some survived but my guess is that the tank wasn't cycled enough. But now I'm thinking it might of been due to lack of dechlorinator too to treat the water. Since he told me the first day he added the fish a lot of them was sucking air. My guess to that was the gills were burning. Again I'm totally new and got most of my info from watching YouTube vids and reading sites.