Do this test in the sump as well as the aquarium.
How to test for electrical current in water.
Salts or other chemicals that dissolve in water can break down into positively and negatively charged ions.
Ec or electrical conductivity of water is its ability to conduct an electric current.
We believe this is from the electrical current flowing into the unit during normal operation and becoming solid on top and then when backwashing the system the bottom became solid also.
Common sources of electricity are going to be powerheads heaters and lighting.
Test the power with a non contact voltage detector to make sure it s off.
The danger comes from a person standing with feet in the water so he is perfectly grounded to 0 volt.
Since the green wire is included in the test at position 3 only the ground current is registered as leakage in that case.
Remember that water conducts electricity so a leaking pump in the display tank will affect the water in the sump because the water in circulation between the two is still in movement.
Before you start working on the hot water heater confirm that the electric power in the room the heater is in is off.
It is the best electrical earth that can be found with virtually no resistance.
Any leakage current flowing in the green safety ground wire would be seen as a non zero reading at position 2 and any leakage current from the load to earth would show a non zero reading at positions 2 or 3.
When we dismantled the water filtration filter the media was melted on the top surface and bottom surface from granular to solid material.
See if electrical current is going to the water heater by touching the tip of the voltage detector to the wires leading into the thermostat.
These free ions in the water conduct electricity so the water electrical conductivity depends on the concentration of ions.