Folks I cannot express how bad an idea it is to "try something and see if you still get shocked". That is absolutely the worst thing you could ever do!! The difference between a shock and fatal electrocution is so small that it can not be predicted. All you need is to contact things very slightly different to have a fatal result.
You need to strive to never place yourself in the position to be shocked!!
The correct way to approach this is to get a voltmeter capable of measuring you local line voltage. 120V AC in the USA.
1) Aquire a meter - borrow or buy. You can get a useful one at the hardware store for under $20.
2) Plug the probes into the correct holes on the meter to measure AC volts.
3) Turn on the meter and stick each of the two probes into the local outlet's two vertical slots. This is to prove power is ON but more importantly that the meter is working correctly. You should see a voltage between 115VAC and 125VAC.
4) Now you need to find a reliable ground as that is where the current is flowing to that is shocking you. Do this by holding one of the probe tips to the center screw of your outlet or into the ground hole (D shaped). Stick the other probe into each of the outlet's two vertical slots. One of them should provide the same voltage reading you got earlier.
If this occurs then the ground screw or ground hole is a reliable ground reference point for further troubleshooting.
Note that the slot paired with the screw/hole that gives the 120VAC reading. It should be the narrow slot of the outlet. If it isn't you should have that fixed.
Moving on.
5) Now to troubleshoot your tank CORRECTLY. You hold one probe to the screw/hole we just confirmed and place the other probe tip so it touches the water surface. You will see the voltage that lurks there. Any is bad.
Now one at a time unplug the various devices that are hooked to your tank WITHOUT contacting physically anything wet or metallic. As soon as the offending device is unplugged you will stop measuring a voltage between the water and the outlet screw/hole.
Remove the offending device and cut the cord off of it as close to the device as you can. This will prevent accidental or even deliberate re-use.
Any questions?