I feed chopped earthworms to my nine goldfish every other day and a good dried food in between, I have never had problems, [ so far ]. The worms are kept in moss [avoiding compost and its additives] for 2 to 3 days prior to use. Fish are healthy and dare I say it positively fat but then they must overwinter in Scotland in a pond, I hasten to point out I am not an expert but do find these little fish interesting and amusing to watch.

The problem I have is water quality. Ponds are both 100 gallons + one flows into other via cascade and pump and filtration, 9 fish 3 to 4 ins long and living plants to remove excessive nitrates. All this being said the water at cascade produces not a foam but little patches of bubbles from time to time and I note that there is a definite correlation between worm feeding and 2 hrs later these bubbles appearing.
At no time have fish appeared unwell they are very active, but I would like to know in advance if I may experience problems.

I have fed my tank goldies earthworms from time to time and I have never had any problems. The bubbles could be by an extra "pulse" of organic matter after the fish digest the worms. I have noticed the same thing in my tank after feeding certain types of food.

I would be cautious feeding earth worms unless you make sure they purge themselves of everything that they eat for at least 48 hours. Worms consume a lot organic matter including fertilizers and such. They also carry several pathogens. If you are looking for a high quality worm food I would go with hikari frozen bloodworms. Much safer

I have noticed the bubbles as well, and wondered what caused that, your explanation works for me. I have often fed my goldfish earthworms from my garden (no fertelizers) and have not have any problems, the fish love them. It is especially ammusing when two fish have the opposite ends of the same worm in their mouths.

I agree. If you raise your own worms in a fertilizer/germ-free environment, it would be fine. Also, as said, put them in a sterile soil for a period of time until they can purge everything they ate/ingested. But, even then, I still wonder. Yeah, earthworms are free whereas buying worms are expensive, but purchased ones are safer. So, I suppose it all comes down to knowing how clean and safe the environment the earthworms come from is.