How deep is the sand? If it's deep enough, you'll grow anaerobic bacteria that will make those bubbles.
I'd tell you to stir the sand, but doing that right now might kill all your fish. You want to eventually stir all the sand every week during your water change. But first, wait until water change day this week and only stir a small corner, vacuuming the cloud of muck as it rises up from the sand and doing a good 50% water change to remove as much of the sand nasty as you can. Then refill and wait another week. Then, on that second water change, stir up the same sand you did last week without fear and also some new sand (that hasn't been stirred) around the area you already stirred, but with more vacuuming and fear. Keep going like this until you stir all the sand every week.
But this advice is based on the assumption that you nave anaerobic bacteria producing toxic gas bubbles. For all I know you have gas in your tap water that is clinging to grains at the top. Tell us more about the bubbles. How big are they? Do they eventually rise and pop? Are there a lot of them or just a few? Are they on top or within the sand?