Under the gravel.
Here's a story (I typically write too much). I set up my 60 gallon tank a little over a month ago. I have one heater stuck to the back like normal, but in an effort to hide some of the equipment, I decided to bury an extra 150W stealth heater (in true stealth fashion) beneath an inch or so of gravel. I figured water circulates enough through the gravel, and if anything, this would help facilitate that process (a la a standard UG heater cable). Poor decision on my part.
My girlfriend called me at work. "Something is happening. The power flickered off and on, and the light on the aquarium made a weird noise, and it was smoking, and now it smells weird."
Many things went through my head. Namely "I just got that brand new Coralife 2x96W fixture in the mail to replace the one that stopped working after 2 months. How bad are these things?"
I ran home, and after some investigation, I apologized to the light fixture, which seemed to be working just fine. If anything, the water smelled a little weird, but not so much the light. I just happened to notice part of the buried heater sticking up out of the gravel. It looked cracked. I unplugged it and pulled it out.
So, my theory is that a power surge (the "lights flickering" in the house) came through the lines, past 2 [cheap] protected strips. The heater, perhaps weakened by repeated expansion and contraction under extra external pressure, couldn't handle the spike, and it popped. I mean literally exploded. It blew itself out of the gravel, and itexpelled some rank chemical smoke, which was caught up by the intake fans of the coralife fixture, giving the appearance of a smoking light.
What's the silver lining? Well, so far everything else is ok. The fish--amazingly--survived. I don't have any RO water on hand, so I can't do an emergency change, but I'll take care of that tomorrow. The light is fine. And the tank is intact. With a pressurized explosion underwater, I hate to think of what could have
happened. For example, the shock wave could have killed the fish or collapsed my log mountain upon them. Or it could have expanded down, blowing a hole through the bottom of the tank, presenting a 60 gallon tsunami upon all of my home theater equipment (which I acknowledge is too close for comfort).
So, all in all, I came through the emergency ok. But I got lucky. Let this be a lesson to you all: use underwater electronics the way they were designed to be used. Don't bury your heater in the gravel.