I have a 200 Gallon FOWLR and sand. I had a 100 gallon and upgraded a month ago. I moved over my old live rock, water, bio-balls and fish to the new tank with 200 lbs of new sand. I waited 2 weeks then added 200 more pounds of live rock. Everything was find for a couple of weeks then starting last thursday we have had a fish die each day. The water has been checked daily with different test kits and by a fish store - water looks fine no Nitrite and no Ammonia - PH looks good. not sure what the problem is why one fish dies each day - up until they die they look fine and eat. Now we have two fish left, we lost two puffers, blue line trigger, and yellow tang...we have clown and a sail fin tang left. the clown isn't looking too good so it will likely be gone by morning leaving just one fish left.
we don't know what the problem is. sounds like some poison or something got in there and test kits can't detect that - but wouldn't know how that could have happened or if so why the fish didn't just die at about the same time.