So, I have never heard of this but listen. I did a water change today in my 10 gallon fish tank. Its got a sand substrate a plastic root object and a piece of driftwood. I have a 100V heater in their due to the fact that my old 10 gallon heater couldnt heat the tank worth crap. The 100V heater i always keep it on low considering it works great. So, about 2 hours after i did my water change. My roomate was in my room sittin on my bed. As i told him to look how crystal clear the water is, i turned my ceiling light off and at the same time I flicked ceiling switch he said he saw a SPARK near the top of the tank heater at the water minimum line, and at that same time my fish next to it jumped. I have never heard of this and i REALLY do not want a fire hazard of any sort, so if u have any advice please give some help!