Mold all over rocks and coral in out tank

[S&R]

We are in need of some urgent help! In three days our tank has taken a turn for the worse and we have no idea why! Our water has turned this murky color. We were about to do a water change. Then we notice all of our live Rocks were covered in this black, like mold (that you'll see the photos). The smell that comes off of the rocks and the coral is incredibly awful.It has already killed some coral And it looks like some other coral won't survive And unfortunately, we have lost a few fish as well.
Does anybody know what this is And/or have advice in how to get rid of it and not allow it to come back.

 

[WalkerLovesTheOcean]

There are no pictures.

Edit: now there are

 

[Oldreefer44]

How long since you first cycled your tank? What are your water parameters?

 

[S&R]

Photos

 

[S&R]

How long since you first cycled your tank? What are your water parameters?

We first cycled our tank 6 months ago

 

[saltcats]

Does it smell like rotten eggs?

 

[S&R]

Does it smell like rotten eggs?

Yes! it's smells like ready bad rotten eggs.

 

[saltcats]

The black colour made me think hydrogen sulfide - rotten egg smell supports that.

The pictures of your tank are hard to tell - did you take all the rocks out? Was that before or after the issue began?
Did you disrupt the sandbed recently?
What is the flow like in your tank?

Hydrogen sulfide is generated in anaerobic areas (with no oxygen) so it tends to form within sandbeds, under rocks, etc.

It is pretty toxic so I'd definitely think doing some water changes to help get that out of the water would be good, and try to increase your oxygenation (add an airstone, point a powerhead towards the surface to get ripples).

 

[WalkerLovesTheOcean]

Sorry, I can't help you with the rocks, but I can tell you that in your tank you have a bacteria bloom. The bacteria uses oxygen as well, so when you have so much of it it can deplete oxygen from the water. This explains the unfortunate deaths of the fish, as they most likely died from lack of oxygen AKA they drowned.

 

[Solo McReefer]

That's is anaerobic bacteria

Scrub as much as you can off with a brush(wire brush), rinsing thoroughly with fresh salt water

Put it back in the tank

The oxygen it is now exposed to, and your CUC will clean the rest off