Hi, If it's a typical UGF then I'm assuming there is a tube rising up toward the top of the tank. If so then the airstone probably is down too far, try shortening the air tubing so that the the bottom of the airstone is well above the bottom of the rising tube. This will also improve the flow rate of the filter as all of the lift is now properly being used in the tube.
If by stronger you mean the bubbles are larger than what comes out of the airstone I would say that this is where the air has "pooled" on the surface of the filter then released all at once.
Hope this helps, I'm basing my response on a few assumptions.