[Æonic]

Are you sure it's not actual static? I've gotten static electricity shocks from water before (altho it was coming from a faucet). In particular, if you do it in the dark and see a spark, I'd bet it's just static. Or you could try grounding yourself good before touching the tank, and if it doesn't happen, then it was static.

(Edit. I wouldn't recommend staying grounded while touching the water, just in case it really is a leak from something).