I did a re-scape, and soon after I discovered a dead Clown Loach

I did a water change and vaccumed half the tank the night before, but I guess I still churned up to much goo....
I couldn't believe my eyes!! The loach was covered in what looked like fuzz that turned out totally slimy!! It also looked kind of backened in the face. Just a few hours earlier they were just fine.
Shortly after finding this one, I moved a large lava rock only to discover a second one dead!! He was floating in a web like goo that was stuck to the bottom rock. He must have wedged himself in there and died

I don't understand how this happened so fast!! How could they get covered in this goo so fast??
I do water changes faithfully ever week and all my water para's kept perfect.
They were the larger two of the three(3 1/2in), the small one survived, to my surprise.
What in the world were they covered in? Was it bacterial, fungal, or is this how they decompose?
I'm totally at a loss and so very sad......
Please help!