Ah my favorite topic cause it always confuses aquarists, and so often is misunderstood/misinformed.
First off, your fish will be fine. Short term effects of ungrounded stray-voltage do not harm livestock. There is even much debate if longterm exposure to ungrounded voltage can harm livestock. Too bad its dangerous to test, hence nobody really does it voluntarily
I'd like to respond to some suggestions here to protect with grounding probes and ground fault devices... By using such "safety" methods you can actually set your tank up to be fully nuked. Here's the situation most reefkeepers try to setup. They first equip their system with a grounding probe, connecting their tank to a ground on a powerstrip, or similar electrical breakout box. Then at the end of that breakout box they place a single ground fault monitor. Their system runs fine for years and years, and the ground fault never trips. Then one tuesday summer day they wake up, check the tank, everything looks fine, and go to work. 2 minutes after walking out the front door, a seal leaks just a bit on their heater and some stray voltage creeps into the tank. When this potential is created, current leaks through the ground wire and into the ground fault monitor, tripping it and shutting the power strip off. The tank now sits with no water movement as the sun comes up. Light enters the tank from the surroundings and the fish wake up and start swimming about. They quickly produce excess CO2 and the tank's pH crashes while the O2 depleats. Fish and corals begin gasping for O2 that the stagnant water can no-longer exchange. It's now about 5pm and most of your fish are near death while your corals are allready on their way there. You run a few errands after work, come home, pick the kids up and whisk them straight away to some extra-curricular, never looking at your tank. By the time you get them home at 9-10pm you walk in the door and smell something funny. Immediately you sprint to your tank only to find it dark and smelling like rotting ocean. You quickly see the GFI is tripped and everything is shutoff. You get the main lights back on and your fish are all dead, corals all retracted, and the water is like a thick cloudy soup. All because the GFI tripped right after you left the house.
Sound horrific? Its happened to a member of my local club. Never again will he use a GFI to protect his system, and rightfully so.
Time for another story. Same scenario, seal fails on submerged power-carrying item. This time, no probe, no GFI. The device leaks stray-voltage into your tank. At night when you feed, you get a good old fashioned zap. Damn that hurt, your hand tingles for the next day or so. All your fish are still alive, and your hand's a little sore. Oh well.
Unfortunately you cannot use multiple GFI's on the tank because a leak to any ground is a leak to all grounds, and the GFIs will all sense the problem and all shutoff... There's really only one method for circuit protection I know that's not perfect but still won't result in a nuked tank. If you first connect your powerheads and sump pumps to the main circuitry, and then upstream from it place a GFI and upstream from that your remaining components, at least that way if the GFI trips due to a current leak, you still have flow and likely a live tank when you get home.
