i have been into tropical fish forty years plus. do water changes hurt fish ? it depends. how much, how offten. lets say, your tank is cured, it has its own chemistry. when you water change, your adding mars to earth. your adding green salt water to cured salt water. to put it simple, like eating a raw carrot, to a cooked carrot. i do only a five percent water change every two, to three weeks. ammonia, is the main fish killer, nitrates, nitrites, are always there, no matter what you do. you can't do enough water changes to get rid of them. plus, you'll change your chemistry so much, your fish will stress out, and die. yes, water changes will kill fish, if done to much. remember, your fish lay in their bed, if you disturb their bed to much, they can't adjust, and will die. well, hope this helps people. i learned the hard way. $300.00 worth of fish. salt water. CHOW !