It is bad!! You have a short in the electrical equipment, most likely a cracked heater. This can be dangerous if you are grounded, or if there is any amount of salt in the tank.
You want a "ground fault interrupter" for your fish tank equipment. This is NOT the surge protector stripes for computers, etc. You can find GFI strips at some hardware store, or you can wire a GFI plug into the wall outlet. <The plug is ~$10, a GFI strip is more.>
GFI is required by most electrical code when attaching any electrical equipment near water.... It trips the circuit if it senses any current, preventing you from being electrocuted in case of a short into the water. This is a must have for fishtank IMO.
For now, i would suggest unplugging everything (without touching the water) & carefully inspecting all cords, etc. Plug things back in one at a time, and measure the voltage across the water & ground with a voltmeter to find the fault. <I would not suggest feeling the water for the shock as some site might suggest!> And get a GFI setup ASAP.