In saltwater tanks, salt creep (salty crust) accumulates on everything. It's an excellent conductor, so the surfaces do not have to be visibly wet. That salt creep found it's way back to something energized.
Obviously, if you are getting a shock, you should not continue using the equipment as though nothing happened.
The first thing I'd check is the motor on your filter. First, unplug and remove the filter. Aquaclear filters have a motor assembly that sort of "locks in" to the filter body. Look for salty crust that has accumulated on the outside of the motor housing. If there is any there, clean it off with a damp cloth. Be sure to completely clean away any residue.
If there is none, try removing the motor from the filter body and look down into the recess where the impeller sits. Look for any cracks or other breakage in the motor body. Perhaps the body cracked, letting seawater come into contact with the internal components of the motor. If you see anything, throw away the filter and purchase a new one.
The other thing you want to check out is your lighting system. Salt crust often accumulates around switches, light sockets, etc, and the current follows the crust anywhere it can, energizing anything along the way. Clean up anything you can find.
Let me know how that goes.
Once you get this solved, we'll deal with the next steps to ensuring this doesn't happen again.
