[mike31154]

Answers will be all over the place due to the variety of equipment we all use. Certain components will induce more voltage than others. It will even vary with the same brand of component due to manufacturing quality (or lack thereof), age of equipment & where/how you have it hooked up.

For example, most fluorescent lights & many MH ballasts are capable of inducing a voltage potential into your water without even being in direct contact. How much will depend largely on the exact type of equipment, how close to the water it is & many other variables. It's next to impossible to have 0 volts potential in your water. Salt water itself is a form of 'electrolyte' and simply having two dissimilar metal objects in the water will turn your tank into a battery capable of producing it's own voltage potential. If the two dissimilar metal objects are close enough together, current will flow between them provided there's a path to ground or the circuit is completed in some other manner. Same idea with any voltage potential in your tank. It's only a 'potential' & will have no effect on anything until a path or complete circuit is provided. This path could be you, if you're standing in a puddle or are wet & in contact with an external ground when you reach your hand into the water. You can also be zapped by touching your light fixture with one body part while your hand or other body part is in the tank water. This is why GFCI devices are pretty much mandatory for anything electrical close to or in your tank.

There are a few things you can do to minimize the effect of electrical gizmos on your water.

Don't fully submerge your heater, control head, cord & all. The manufacturer may claim it's fully submersible, but are you willing to take their word for it with your life, your family's life or your home?

Keep the number of power bars to a minimum, ensure you have drip loops on all the cords & keep the outlets as far away from water as practicable. Ideally you will have a wall receptacle for each component you use on your tank and each one will be GFCI protected. If you read up on codes, power bars & extension cords are only supposed to be used 'temporarily' with moveable equipment. Most of the stuff on our tanks is hooked up 24/7/365, so hardly 'temporary'.

Use external return pumps instead of submersible pumps. Change your AC return pump for a DC model. Same goes for power heads. If you can afford it, go with low voltage DC powerheads like VorTech or the low voltage Jebaos now available. The fewer cords & motors submersed in your tank, the safer & less prone to induced voltage. AC voltage due to the constantly expanding & collapsing magnetic field, will always induce more voltage into surrounding material than DC voltage. And since household AC is up there around 120 volts whereas most DC components available are 24 volts & less, the safer choice is obvious.

Ditch your T5HO flourescents & MH lights for LEDs. There remains much controversy over what grows the best SPS & what looks more appealing to a given hobbyist's eyes, but if safety is the driving factor, low voltage LEDs are the way to go. Fluorescent & MH ballasts produce voltage potentials of up to 1000 volts or more, depending on the fixture. That's a scary number around salt water.

3 or 4 volts won't hurt a thing.

I personally don't use a grounding probe. To me it's a waste of time/money & is a sure fire way to provide a path for current to flow, which is the last thing I want to do. If you find a voltage in your system in the 30 range & over, starting looking at your gear & fix whatever's causing that amount of voltage to be present. You pretty much did the right thing in identifying your maxijet as the culprit.