Just doing some tests with my multimeter. With one lead in the tank and the other in my hand, multimeter set to AC here are results...
Total Difference
No power(RCD switched off): 0
Power on, everything off: 3 3
Return pump turned on: 6.3 3.3
Sump lights: 9 2.7
MCE600 skimmer: 16.6 7.6
2 heaters: 25.5 8.9
1 seio: 28.2 2.7
2 Seios: 30.1 1.9
My DIY topoff turned on: 86.9 56.8 Very definite tingle
I don't know if where in the series any item is turned on effects the voltage difference, but I assume it's not going to make a bigger difference than the obvious problems of the poor ground caused between me, socks, carpet and the concrete floor. This was trying to stand in the same place each test. Obviously to a good solid ground would be better, but I couldn't get the probe into any of the ground prongs on any outlets.
Needless to say, I've turned my topoff off. I've been meaning to make a 12v relay system for it. Maybe I shouldn't have procrastinated so long.
*Heads off to jaycar.co.nz*