freakin UGF creates massive bubbles!

you problaby have too much air going through it or the pumps outlet is too low, and yes you would need a diffuser or airstone on it, try it with the stones and see if it stops. plus you need a thicker layer of gravel and no open spots for it to work right. if your whisper is working fine we would all probably recommend you to stop using the undergravel, its just pulling all the debris under the plates which will accumlate underthere.

actually big bubbles are better than small ones. They move alot more water. In airlifts people try to get the largest bubble size possible to move the most water.

You could always go RUGF... But I like my HOB's. I would never convert, unless maybe I got a really big tank, then I would go canister.

UGF's,
I am a big fan but they are easily mis understood, and often lead to problems. Your LFS should have explained things better, but I'm not suprised. You mentioned you didn't understand the need for one, to be brief, it creates a bacteria bed to biologically filter your tank. Gravel should be more than an inch thick over the entire plate. The RFUg mentioned above is far better than airstones, Reverse flow is done with a powerhead, and pushes the water down the tube and up through the gravel. this prevents build-up of dirt under the filter and keeps you gravel much cleaner it also feeds more oxygen to your bacteria and therefore they work better. Be sure to put a sponge filter on the intake of your power head to prevent dirt from buliding up under the plates. If you use the pump and airstones, try raising the stones an inch or two in the tubes to keep air from going under the plates. You are really only trying to create water flow with the air, as the bubbles move up the tube, the water is forced to do so as well. If possible, try to get all the air out from under you plates as the filter will work better without trapped air. On a 10 gal I wouldn't call an undergravel necessary if you have a good HOB but I never set up a tank without a UGF installed.

The bubbles need to come out ABOVE the plates or some bubbles will gather under the plates. Move your air hoses up some.

Someday you may want to try different types of filtration....
Good luck..

Pull the airline up farther in the tube. Had the same problem, the air stone was putting air under the gravel and it would accumulate on an uneven side of the tank (old house) where when it got big enough it would rise to the surface regularly in a big bubble scaring the crap out of everyone in the tank.

I disagree with the idea that large bubbles move more water than small. IME it is exactly the opposite.