You do not have stray voltage. You have a ground fault. This is a dangerous situation that must be fixed.
Using a grounding probe when you are getting a shock from the tank is only going to make things worse. It's attacking the symptom and not the cause.
First and foremost, all of your tank's equipment should be fed through a GFI. A GFI would likely have tripped the moment you touched the water if not beforehand. It's a critical safety device.
Using a grounding probe without one can make a dangerous situation even more dangerous.
The suggestion of unplugging one piece of equipment at a time until the problem goes away is certainly one way to approach it. You must find the problem, not just hide it with a probe. Of course you need to "get shocked" to do the test, and not getting shocked doesn't necessarily mean you fixed it either.
What I'd do is get a GFI unit and plug your tank's equipment into it. You can get units that simply plug into the wall receptacle, then plug your tank into it. Or, you can replace the wall receptacle with a GFI variety you no doubt see in your kitchen and bathroom. If you aren't electrically inclined, the plug in variety is what I'd use.
Then put a grounding probe in the water. Home Depot doesn't sell them, but most dedicated fish stores do. The GFI will likely trip instantly when you hook it up.
Then simply unplug one of your pieces of equipment, and reset the GFI. If it trips, try something else. Once the GFI holds, you found your defective piece of equipment. Replace it and you should be good.