That sucks, everyone has had a crash at one point, it is very frustrating. How long were the fans in your water plugged in? If they were in there a long time electrolysis might have started to take place. Chlorine gas might have started to form in the water and poison the inhabits . Is the water a different tint? If so change all or the majority of your water. The electricity from a 12V fan shouldn't have killed your corals.
This video I found on 'utube' demonstrates the principle.
Lots of other nasty stuff could have been made as well depending on what metals are present on the electrodes of your fan.