I found this
while wandering around Youtube. At first I thought it was some kind of dissolving rocks. But upon closer inspection, it seems the guy put some concentrated CO2-producing stuff in a bottle, weighted it with rocks, and then stuck it straight into the tank. Any ideas what might be in the CO2 mix? The title says gel, but it looks like normal yeast-and-sugar-water to me, albeit a small quantity for the amount of CO2 produced. Maybe my first impression was right and the rocks are limestone and the liquid is some kind of really strong acid - horticultural vinegar, perhaps?