Testing for voltage, heater causes a 0 reading

[fundee]

Hello, I’m attempting to test for stray voltage and came across some interesting behavior. When I have nothing on in the tank, my multimeter reads nearly 0. One ac wave pump reads around 1.5, two around 3, but when I then turn on my heater I get a 0 reading. I’m in the middle of a big water change so can’t test for everything this moment (waiting on more water), but does anyone know how that could happen? I get an exact 0 reading when the heater is on, regardless of what other equipment is running. Although I havnt been able to test yet with larger pumps on. I should by this evening be able to provide more details. Very curious, any thoughts are appreciated!

 

[WVNed]

Some heaters have 3 prong plugs and act as grounding probes