The tank works "air-over-vacuum." As juice is brought to the coil via the wick, the juice in the tank is used up. Since the tank is sealed, lowering the quantity of juice in it lowers the pressure as well, drawing a partial vacuum. The chamber-- on the other side of the wick-- is at ambient pressure through the drip tip. So air is forced through the wick into the tank to replace the volume lost by using juice. There are the bubbles you see, and it is perfectly normal. All tanks do this, except for top-coil Genny tanks.