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Hi BC,
I am an electrician,The advice these guys gave you was right on,this should sound an alarm.The GFI in my opinion is working properly,it is doing its job.When a breaker trips,it either trips on overcurrent or on dead ground.The GFI is designed in such a way that it monitors leakage from the hot,neutral to ground.Any leakage or passage will trip it.
A GFI is very sesitive and will trip way faster than its circuit breaker.
A GFI can trip on humidity,where the humidity makes it to the wires and thus creates a path from one side to the other.
In many houses GFI are installed outside in the carport or garage,they often trip because of humidity.My guess is that a wire or equipment has its connection exposed to humidity,somewhere.
**** Do not attempt to put your hands in or anywhere around a wet part of your tank without having installed a ground probe,and wiring it dirctly to the GFI.

The GFI is giving you a warning...take it as such.
People get killed every year from electrocution.

Do not rely on the GFI to save your life,put it this way,what happens if the GFI does not trip?And you attempt to put your hands in the water.
Once had a problem with powerheads,they leaked a voltage of 135 volts into the tank.When I put my hands into the tank I got a stinging sensation.Sort of like static.When I installed a GFI it tripped.

If you have a voltmeter put 1 prong into the water of your tank and the other prong onto the ground of your receptacle(Wall outlet)It should read 0 or close to it.
If you get a voltage reading something is leaking voltage.Turn off equipment 1 by 1 until you find the culprit.
Being an electrician I can tell you first off almost all magnetic motors will leak...
Powerheads...Pumps...
Is your heater in good condition?
It's not easy but you can find it.
Good luck.
Dan