My wife and I went on Holiday for 2 weeks and came back to a disaster. My 18yr old nephew looks after my 3 tanks when we go away. Came back and all my fish in my 75 gallon were looking bad. Flame Angel died, six line wrasse died, Anthias died, Royal gramma died, flame hawkish died, 10 snails died. Surviving fish 2 Cardinals, yellow tang, 3 osc clowns, scooter blenny, 2 bubble tip anemones, long spine urchin. My skimmer was bubbling like no tomorrow . There was some string like jelly hairs floating in the tank.. Water tested fine only high reading was nitrates at 20ppm which is not bad. He put prime in there every 3rd day just in case anything got high. Had to move survivors to my 55gl tank. Seems like some type of chem might have been introduced into tank.or a bad parasite thing I'm at a loss. We did 2 95 percent changes and the fish all were breathing heavy Still that's when I put them into the 55. Now they all seem fine in the 55. If my tank is tainted do I need to trash all 100lbs of LR and 75lbs of live sand and start over? My wife and are Very saddened and at a loss. Please need suggestions. Thank you kindly in advance.