Sounds like you need to get yourself a multimeter
Some things though:
GFCI trip when there is a large enough difference bettween the positve side and the negative side.
If there is no good ground in the tank, the stray voltage has nowhere to go and wont do anything, untill a ground is introduced(i.e. you)
Electricity ALWAYS takes the shortest path with the least resistance. It may be leaking through the stand or still going through the negative side on some cord, but the second that some path in introduced thats shorter or has less resistance, thats the way its going to go.
Electricity travels in a fairly strait path. If you have a gounding probe in the tank and the device is still leaking electricity, its going from that to the probe and any thing that gets in its way it will go through(i.e. fish, corals, you).
If your house is not grounded well, or not at all, then DONT use grounding probes, because there will be a good chance that you will become the ground for the whole house(trust me, its not fun
)
Hope this helps, and if I am wrong about anything some one please correct me.