This is a very informative link about using Sand as your substrate. You'll read about the toxic gases it can produce right here: http://www.fishforums.net/index.php?showtopic=131373
The very first sand that I had used, years ago, well, I hadn't heard about the gases. I made it deep with little hills here and there. Well, one day as I was feeding my fish I smelled something so awful. It smelled like a rotten egg. Like SULFUR. It was my sand! I quickly removed more then half of it and did a water change. whodathunk sand could cause this? That was play sand.
The very first sand that I had used, years ago, well, I hadn't heard about the gases. I made it deep with little hills here and there. Well, one day as I was feeding my fish I smelled something so awful. It smelled like a rotten egg. Like SULFUR. It was my sand! I quickly removed more then half of it and did a water change. whodathunk sand could cause this? That was play sand.