I would be concerned that there's filth in your water. If you've ever been to the beach you've seen parts where the water was very foamy while there were other parts that weren't, the foamy parts was 'natures cleaner'. I do this on purpose with my fish tank. I have what's called a Protein Skimmer, it cleans the fish shit out of the water as well as other contaminants.
An air stone will cause the same reaction in most fish tanks. There are two things I would be worried about (with exceptions.) First I would be worried that you have an excess of salt built up in your water, the action of foaming occurs much easier in salt water than it does in fresh water. Second I would worry about the water having contaminants that you might not want in your plants.
The exceptions are if you are using something like an organic nute system, it's possible that the fecal material in organics is causing the foaming. The other possibility is that you're using a strong chemical nute that uses salts to deliver the nutes to the roots. Once the water gets even a little salty it will start to foam.
I've been running a salt water tank for 5 years and a freshwater tank for almost 13 years. The protein skimmer is VITAL to the health of my salt tank, but don't use it in the fresh water as it's much less functional.
Here's an article on the reaction thats occuring
http://en.wikipedia.org/wiki/Protein_skimmer
-RT76