I have never used a disk so it "may" need to soak water before it sinks, but not likely. You can get airline holders to help. They clip around the airline and have suction cups to hold them to the glass. Put one under the gravel with your air supply line attached and stick it to the bottom glass, then bury it with gravel. Another option is rigid tubing. Instead of the flexible vinyl hose it is a clear acrylic tube, so it does not bend. You can use this as a "riser" and attach flexible tubeing to it. Or you can combine both methods. More than likely if the airstone somehow becomes heavy from water saturation, the air in the flexible tubing, with no suction clips will become buoyant and float to the top.

Mine did the same thing...I found a rock with an indentation on one side and laid it on top of the line.
Worked fine till my fish discovered it...they can move mountains! LOL
I finally switched it out for a regular wand-type.