Hi
Yes, I have found earthworms to be a natural laxative for garters.
Goldfish are a problem food, but only if you fed them frequently. It is best to avoid goldfish. Acceptable fish feeders for garters include the following:
Rosey Reds (small pink minnows sold in pet stores)
Bait store minnows called "Crappie" minnows
Strips of trout
Guppies (sold as feeders in pet stores)
I would suggest freezing all fish for 30 days before feeding, thaw the fish in a container of warm to hot water, then place on a dish with a few drops of water around them to keep them moist.