I know this sounds crazy, but check for any electrical current in the water. I have had people with massive reef systems lose all of their fish due to a heater shorting or something. Salt creep is a great conductor, so it can happen over time even when everything is working right. And the funny thing is, almost every time I have seen this, the inverts are fine. ODD, I KNOW! Give it a try. If you don't have equipment to test it, try this: If you have any exposed scrapes, abrasions, fingernail chewed down too far, anything like that, Unplug everything, put exposed wound in the water, and plug everything back in one at a time. You will feel the slightest bit of current right away if there is any..... It's worth a try...