Matt - so sorry to see this.
I sure hope we aren't seeing you on here right now because you are hopefully doing something to save what is left of the tank. As someone else on here mentioned - there are quite a few pieces in there that look like they can very well be saved!
If you need a bunch of water (and can't make it fast enough), I'm sure many here will offer some up. Also good idea to give WWoF a call, they have many tanks with nothing left in them and I'm sure he can spare some water.
As for the 'why' part of what may have happened...
As Robin quoted in her post (and for anyone else who wants to use Red Slime Remover in the future) - it is CRITICALLY important that you Oxygenate the tank
thoroughly before & during use of the product. IMO that means - start DAYS before by adding an airpump & airstone.
Another thing that MUST be taken into consideration is water volume - not the size of the tank. I see you have a 90g tank - but with all of that rock in there you do not have 90 g of water (unless you are running an extremely large sump).
While I hate to say it, you may have overdosed the tank as you used a dosage for 90g of water
If there is anything I can do to help - please contact me, I will PM my phone #.