You need to test everything by unplugging them one at time, heaters, pumps, and even lights, any of them could be the problem. Although heaters are notorious, and I have had that problem with titanium type before, a hanging light cord in touch with water can also be a problem.
It can also be helpful to get a grounding probe, these plug into a grounded socket and help absorb excess current.
http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=3946
It can lessen to shock enough to save you from having heart failure.