My aquarium water is very slightly foamy - what could be causing this?

My tank gets a little foamy when my water is starting to get low because the water the filter puts back out starts to get higher above the waterline causing more aggitation which causes a lot of little bubbles that build together creating a light foam. My bubble stone also causes a lot of bubbles to build up but these are usually a little larger and pop faster. The bubbles don't hurt anything, just add a little more water to the tank.