I occasionally feed my corns fish, and here is what I have learned about it.
1. Goldfish contain an enzyme called thiaminase. It destroys thiamine (vitamin B1) so even if you supplement them it will still be destroyed. Snakes can get a deficiency from being fed goldfish too much. Some symptoms are loss of co-ordination, head waving, balance disturbance, anorexia, weakness, and finally convulsions and death.
2. Most fish from pet stores have parasites or are very likely to have parasites.
3.Thiaminase can be destroyed by heating the fish in 180 degree F water for five minutes. This would probably also kill parasites, but I'm not 100% certain about that. It does make the fish very messy to feed though.
Why do I feed fish? Well, I raise tons of guppies. My culls go to my garter snakes, but occasionaly there are too many for the garters to handle. Since they have been in my possesion for several generations, and I was very careful to deparasitize the parent stock I feel that the risk of infection and parasite transmission to my snakes is minimal. Also, the guppies make up a very small portion of their diet. About once every 1 or 2 months. The rest of the time they eat mice. So far they have suffered no visible ill effects, and are growing at the same rate as the corns that refuse fish.
Some of my corns love the fish and have fun "stalking" them. There are a few that are like "what the heck is in my water bowl!?!" and some that totally ignore them.
Personally I would avoid goldfish, and avoid feeding them fish from pet stores.