I just had this exact same heater blow and take on water, I'd used it for about 18 months. Thankfully I had just moved it to my salt mixing container bc I replaced the heaters in my reef tank. Turned it on and heard popping from within the mixing container. Definitely confirmed I was right to just replace heaters before they fail....phew. Also, though the heater is connected to GFCI I will now be also adding a grounding probe, that popping really scared me.