About two weeks ago, I bought a potters wrasse. It died after about 4 days looking as if it was having trouble breathing. He was replaced with a blue sided wrasse that lasted a week. He looked as if he was having trouble breathing as well. All the other fish are fine. There is no sign of ammonia or nitrite in the tank. Nitrates are at 30 witch is isn't terribly high. I just did a water change on all 3 of my tanks and they are all fine and there has not been any fighting either. Any ideas?