If you find yourself getting shocked every time you put your hands into the aquarium, you are experiencing stray voltage. The best method of attack is to unplug everything, following by plugging things in one at a time. Or, leave everything plugged in and unplug everything one at a time. Eventually you will find the culprit, which is usually the aquarium’s lights or a powerhead. If you cannot find the source of stray voltage, you could ground all of your electrical equipment or use a grounding probe in your aquarium.