Ditto, you need to look for places where there may be either
salt creep or wet surfaces near the socket or the wiring junctions for the load. Pay special attention to any junction boxes or locations where there may be wear and tear on the insulation for the wiring (like at hinges, etc.). Use a meter and check for voltage between the water surface and the ground for the system as well. If voltage potential exists, then unplug each assessory item for your aquarium one by one until there is a drop in voltage on the meter. That will be where you're experiencing shorting between the particular device and the water column. Replace the item with a good fault-free device. GFCI plugs somewhere in the branch would be smart.
Just a safety suggestion: isolate each power branch from which the aquarium draws power with an arc-fault breaker in addition to GFIC.
HTH