Aquariums: My tank’s uptake tube (undergravel filter) seems to push airstone bubbles DOWN as much as up.

Hi,

I use an undergravel filter in my 40g tropical tank, with two uptake tubes connected to powerheads and a third using an airstone to create surface agitation. Over the past few weeks, the airstone bubbles are piling up UNDER the stone (closer to the bottom of the tube), which means that there’s a loud BLOOP about every five seconds as the bubbles combine and force their way up to the surface.

I’ve changed airstones but see no change. This tank’s setup has been going steady for nearly two years, and I can’t figure out why the airstone is doing this now. Does anyone have an idea as to what’s up?

Thanks