IMO what ever happened caused the oxygen level in your tank to fall with the resultant shock and fish deaths.

With any type of material change that sees fish gasping, IME it is a good idea to get as much oxygen in the water as fast as possible.
This can be accomplished by using air diffusers, adjusting the skimmer and placing a large powerhead a couple of inches below the surface of the water and a large water change

Sorry to hear about everything Beverly. Hope everything works out for you.

This worries me as I have some black patches on the bottom of a small piece of LR I bought from the LFS last week?
I thought it was kind of weird seeing black patches...hmm... not sure if this is bad or not.

If it is uncured LR it's probably dead stuff.

Anyway, I have an update on the tank. Read an article on the causes of cyano. Old lighting was a big factor in cyano growth. So we found the box of VHO tubes we'd ordered shortly after we set up the original tank about a year and a half ago. The box was full :eek: So the very old tubes were replaced today :redface:

We're also planning on doing another 40% water change on the weekend and hopefully get nutrient levels very low, even though 2-3 weeks ago that tank only had 5 ppm nitrate.

Will also be doing some serious water chemistry testing, which I think has been another factor in cyano growth. Have pH raising chemicals, liquid calcium and magnesium along with all their test kits to make sure chemistry is up to snuff. Have been neglecting the chemistry of my tanks for the past several weeks. This cyano problem has kicked me in the butt to get everything sorted out. Kalk is brewing as I type and will included in my daily tank maintenance regime again.

BTW, after adding calcium, magnesium and raised the pH in all three tanks today, corals have perked up considerably. Still have to make sure I added enough of everything, so testing begins tomorrow :exclaim:

Bev, I'm not sure if salinity has much to do with ammonia levels but pH does, if your pH dropped rapidly ammonia wouldn't have been a problem however if your pH went up for some reason, ammonia becomes more toxic as the pH rises. Any chance there was dead material under the "cyano" that could have released some nasty gasses? I think a pH crash would have occured through but I'm not sure. That being said, it's a large tank so I would assume you didn't remove enough crud to do that. If those snails had died under the rocks they could have been the problem, and when a snail dies there is lots of black crud as they decompose remember they are almost all protein and they decompose fast.
Glad to hear things are clearing up. Here's to the end of disasters in the reef tanks. (clink)

Doug

Doug,

Was not monitoring pH during the event. I am assuming that pH and alk were both low low for awhile preceding the event because I hadn't been monitoring my tanks very closely :redface: This situation is now in the process of being remedied.

The dead snails were the result of the event and did not show up until the fish started dying. and in the days following. They were victims of the event, rather than contributors to it. The dead snails were removed daily.

Also, don't think the green algae beneath the cyano was dead, even though cyano covered much of it. There is an east facing window behind the tank and a healthy growth of coralline on the glass. The green algae is now growing back in a few places.