Do waterchanges with RODI mixed water until it's 100% changed. When using GFO, don't use a stocking, doesn't work all that great, pony up for a reactor, and make sure it's set up properly. You can get away with carbon in a bag, but GFO doesn't work nearly as well in a bag, as it does in a reactor. Kill lights until problem is corrected. The fish will be ok. Don't add coral until the big 3 are stable. Sure sign of a stable system is coralline algae growth.