There is a problem with certain fish species, Chris, Goldfish for example. However, there are many types of fish you can safely use. I would recommend that you vary the diet as much as you can...fish, pinkies, worms. Have a read through this link, it's all about the problems associated with feeding certain fish to Garter Snakes.
http://www.thamnophis.com/caresheets...C_.26_GOLDFISH
I would also recommend this;
http://www.thamnophis.com/caresheets...ake_Care_Sheet
as a bit of general reading.