Tank, my FW
sand does the same thing, its under the rocks, I removed over 18 rocks due to them turning black themselves, like jet black.
I stir with life as well as a hair pik super glued to the end of my gravel vac,works best for cleaning sandbeds.
I was told it was bad stuff, I get bubbles from mine as well 55g been set up since two weeks before fathers day.
If you are worried about it, take a chop stick from the side, put the gravel vac into the sand, stir with stick under the vac, any bubbles will be directed out of the water column, just dont breath over the end of the hose, the bubbles it does release smells like poo.