Can't say for certain, but I don't think your coral problems stem from stray voltage. 20 to 30 VAC is not overly high. Besides, unless you have a ground probe installed, the voltage
potential that you are measuring has no place to go until something (like your meter or a body part) completes the circuit between the water and ground. Otherwise it is just sitting there as a
potential. If the circuit is completed by you or a grounding probe, then current will flow, the amount being dependent on the existing potential and the resistance of the path it takes to get to ground.
Aside from equipment submersed in your system, your lighting can actually impart a substantial charge to your water by electromagnetic induction. Especially fluorescents. Until I grounded each reflector on my T5HO retro set up, I would get a nice little buzz whenever my arm brushed against a reflector with my hand in the water. Other than this buzz, there should be no harm to the system inhabitants since there will simply be a potential difference between the light reflector and the water, so no electrical current flows until the circuit is completed, again, in this instance by my arm. Even then, once the current is discharged to the water through my arm, it has no place to go... unless there's a grounding probe to allow the circuit from the water to ground to be completed. Before grounding the reflectors I was measuring well over 30 VAC between them and the water, don't recall the exact number. As you can see, things can get pretty complex and confusing in a hurry.
Here are two links to articles by a Robert Michelson, Principal Research Engineer, Emeritus - Georgia Tech Research Institute. I think it's fair to assume that this is a trustworthy source for this type of info.
http://angel-strike.com/aquarium/GFI...alDetails.html
http://angel-strike.com/aquarium/GroundingProbes.html
I try to keep the electrical gizmos physically in the water to a minimum and run my system on two separate GFCI protected circuits. Only things I have in the water are two heaters and a small powerhead delivering water to my skimmer. VorTech pumps provide flow (electrics are on the outside of the glass) and I have no sump, so no return pump at this time. The more low voltage DC components you can use, the better. That way, electromagnetic voltage induction is virtually nil and safety of you and your livestock is greatly enhanced. The future is looking good in that regard with more LED lighting systems on the horizon as well as an increased selection of DC voltage powerheads and pumps.