1. It's 1/4" thick- how much detritus would need to migrate through 2-2.5" of sand I'm using to cause a problem in a 1/4" deep pocket in a regularly maintained tank? I do a full break down on my tanks on about a yearly basis for a thorough cleaning anyway- it's a carryover from years of using UG filters (which I never had a problem with).
To me, 'smelly' usually means neglected or improperly maintained.
Depends on the sand depth and the species. Many dig, as I'm sure you know and help the process of 'migrating'. I've found that certain cichlids can fill a sand bed with organics pretty quickly, like tropheus or 'l'. brevis. Tanks can be maintained same as others, but have an organics issue to deal with. Doesn't always mean neglect. Keep 'l' brevis in a tank for a year without touching the sand and you'll see what I mean. I've had other tanks with certain species where I didn't have to touch the sand at all, ever.
Some thoughts and questions about pressure points. I don't have answers to all of these questions, so just something to think about.
No one would lay a rock in a tank on a bare bottom on its pointed side, so I"m assuming we're talking about a grain of sand getting under the rock and creating a pressure point. How much pressure would it take to blow out the tank bottom? How much pressure is typically exerted by a rock and any rock(s) stacked on top of it? What would be the max possible pressure of a stack of rocks, realistically? How many people stack rocks more than a couple of layers high? So, would the pressure ever come close to being enough to blow out tempered glass? Rocks are somewhat buoyant, so consider that also.
I know there are tools made to blow out car windows, etc, but these are 'impact' tools, so not what we're dealing with.
If the rocks are laid in the tank before sand, and other rocks are stacked high enough to create this pressure, then how did the grain of sand work itself under this pressure? Rock shift? Maybe, but then isn't there then going to be more than a single grain of sand under that rock? Or maybe there's that one big grain of sand that the rock shifts over onto? Is a rock pile with that much weight (a few layers high) going to shift?
What about a single large rock, say 50# with some points on it? The entire 50# wouldn't sit on one point. The pressure would be much less on any single point. So, is 10, 20# or so enough to blow out the tank bottom?
Tempered glass is somewhat flexible. For a pressure point to blow out tempered glass, it needs to be applied at a point where the glass is unable, or least less able, to flex. So, the pressure point would need to be nearer the edge where the glass was not as able to flex. Most rocks are stacked more toward the middle.
I just think the odds of a tank blowout from a pressure point is incredibly unlikely. The fact that we're not hearing about it happening here at CF seems to confirm that. Or maybe everyone here is using eggcrate?