It doesn't sound like you're using a GFCI receptacle, or a GFCI-protected circuit. Please fix that. They're designed to trip when they lose around 5 mA to ground, which is happening when you get shocked. It's also possible that what you're feeling is below the 5mA threshold, and the GFCI hasn't tripped yet, in which case, well done on using a GFCI.
Most aquarium devices do not not have a 3 prong plug, lacking the ground.
GFCI's ground is also not in contact with your water to detect current leakage.
The water may become energized, depending on how well you are grounded will determine the shock factor.
But in a 3 phase system you'll never have 240 volts. You'll have 120/208 or other normal voltages.
This is only true in a "Y" configuration.
240 and 480 delta configurations exist with 2 live phases and a bonded center phase.
Meaning 240 or 480 to ground from any phase.
High volt phases also exist, maybe as high as 192 volts.
Don't plug a drill in that one, it will spin very fast then smoke.
It is said that 200mA can cause cardiac failure in susceptible individuals.
That is only 1/5th of an ampere.