You have too many dissolved organics in your pond water. That means you either:
1. Have too many fish, or
2. You are over feeding the fish you have, or
3. There is a bunch of gunk at the bottom of your pond decomposing.
It could be any combination of these, so first let's look at your feeding habits.
Do you WATCH your fish eat when you feed them, or do you just toss in a handful of food and walk off? It's important to watch your fish eat. You will be more likely to notice if there's anything wrong with them during feeding than any other time. Also, you only want to feed them as much as they will eat in 5 minutes. If food is left floating around uneaten, it will settle to the bottom of the pond, decompose and foul your water.
So you're not overfeeding them. Let's check the bottom of your pond for decaying plant materials. Do you have spent lilies that are rotting beneath the surface? Remove all that filth and do a major water change, or series of 10% - 20% water changes over a period of several days.
Filth all gone? O.K., how many fish do you have in your pond? There are a couple of formulas for how many fish you can have in a specific volume of water. The first, and most popular of which states something like '1" of fish for every 10 gallons of water', or some other malarkey. Obviously, one 10" fish is going to produce A LOT more waste than ten 1" fish. The second formula incorporates POUNDS of fish per gallons of water. Neither of these equasions take into consideration what type of filtration you are using, so although they may be a good "generalization" of what to expect, they are by no means precise. If you started out with ten 1" fish and now you have ten 10" fish, that may be your problem. If you don't want to find any of your fish a new home, you can increase your filtration and do more frequent water changes to help permanently solve this problem.
Now that we have discussed the probable causes of this bubbly problem, let's try to fix what is wrong right now and work on your filtration and fish load later.
First, you guessed it, do a water change. If you can, change the water from the BOTTOM of the pond. This may take some ingenuity, but you should be able to rig up a siphon to suck the worst water off the bottom. Remove and replace about 20% of the pond volume, and remember to treat with AmQuel if you're on City water.
Next, you may want to give a product called TerraPond a try. This is an all natural montmorillonite clay that comes in either a powdered or chunk form. Montmorillonite clay is not only good for your fish, but it's ions are negatively charged. Since the ions of organic waste are positively charged, the montmorillonite clay particles will attract the organic waste particles and bind them up into bigger particles, which will sink to the bottom of the pond to be removed by your filtration or manually.
TerraPond is listed on our products pages under "Foods and Other Nutrients". Of course, you can call or e-mail me any time if you have any more questions.