I have a Redsea max s400 up and running almost 3 months. I originally had a biocube 32 with 6 fish. 1YT 1 Blue tang 2 clowns 1 orchid dottyback 1 YWG all are in new tank and doing fine. About 2 months ago I started adding additional fish 1 at a time I’ve been getting the fish from live aquaria. all have died within 2 weeks. They have included 3 flame angels 2 basslets and a McCoskers wrasse. My local reputable LFS was thinking it is due to stress from shipping etc. so 5 days ago he gave me a very healthy key hole angel. Since then he has been doing great. Then yesterday i noticed a very long stringy white poop hanging off him. Today he won’t eat. Is it possible i have some kind of parasite that original fish are immune to? Tank parameters are really good. 0-ammonia.03 phosphates and 4.5 nitrates. Any help would be appreciated