Electricity + Saltwater = Please Help!

[Stege_saurus]

I had 12-24v. Did the same thing, unplugged everything... Its induced voltage and you wont get rid of it without sending it back to the grid - as others have said, get a grounding probe - solved the issue for me (but I'm paying for 24v that I dont get to use!!!).

Try your heater but make sure its on (heating)

Electrician here.
It is not uncommon for your T5 ballasts to produce eddy current voltage to ground.
That said I don't see that stray voltage getting into your tank unless your grounding system is not adequate overall.

I also doubt that a 5V reading could be felt but in the world of electricity I have found anything is possible.

I will agree with everyone else that you should unplug ONE thing at a time until it goes away. So unplug and test, plug back in and move to the next repeat.
If you unplug something that is NOT in direct contact with the water like your lights and the voltage goes away then your total grounding system has a serious fault and needs a professional to check it out.

First and foremost, thanks for all the help. What I have found:
Tested the water with the multimeter, found no current going through.
Forced my heater to kick on by pouring cool water over it, it kicked on and produced upward of 20V directly into the water, and cause an additional 50-60V surge at the outlet. I'm getting rid of it and going to get a new one before the store closes!