Another sad tale

[howdy20012002]

Well, I went down to LA last weekend.
I left friday afternoon
When I called on saturday evening, I was greeted with the news that my entire seahorse tank had castrophically crashed, killing all 12 of my beautiful seahorses.
Apparently, They were all huddled together in the corner as if they were trying to console each other.
The frustrating part is that I don't know what caused the crash.
Unfortunatey, I was unable to test the water to see what the chemistry was like at the time. When I got back, the parameters were right out the roof because a couple of the seahorse bodies were hidden within the rock.

I had moved a couple of things around in the tank on friday, scrapped some algae scum off the back of the tank and moved a powerhead.
But I don't think that would have caused the crash.
Even if I lost one seahorse, I don't think that it would have caused the tank to crash because I had lost one in the past and the body was gone within hours by the scavengers.
Any suggestions as to what caused this? could the algae that I had scrapped off released something that killed them off?
I know seahorses are picky, but besides losing the one a couple of months ago, I had no difficulties at all.
As I think seahorses are awesome little creatures, I will be getting more and was just wondering how long should I wait before putting more in the tank? Obviously I will be testing before I put anything in. I also did a 100 percent water change when I got home.
Any help would be greatly appreciated