I use a Fluke pen to test for A/C voltage on wires. It looks like an a fat plastic pen that just needs to be close to the current and then lights up if there is power there. I haven't tried it in water though and without purposely adding electricity to my tank I can't say for sure if it works. They are cheap and other companies besides Fluke make them. They come in handy for wall receptacles, testing extension cords, etc. If you know any electricians they would be likely to have one to try.
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