Hi Jennifer and welcome to the forum.
Fish, in general, aren't bad for snakes at all, but certain species can be harmful. Have a look at this article for more details (
http://www.thamnophis.com/caresheets...?title=Thiamin,). The bottom line is that your snake will be just fine without eating pinkies.
A 10" garter is not more than several months old, so this is its first winter. It's common practice for people keeping snakes NOT to brumate newborns, even though it would happen naturally in the wild. Your snake will grow faster if you keep feeding it throughout the winter. If you decide to brumate it, its stress level will likely remain lower, making it a better candidate for release in the spring. If you don't, you increase the likelihood of causing at least temporary stress, which depresses the immune system and may make your snake more vulnerable to any pathogens that it may currently be carrying. Therefore, releasing it in the spring (without brumation) potentially allows pathogens to multiply, turning your snake into a "Typhoid Mary" by spreading pathogens to every other snake it comes in contact with.
Some states have laws against releasing captive reptiles, but for the most part, it's a personal decision that you'll ultimately have to make. If you decide to keep the snake active, just clean its cage regularly with 10% bleach, and enjoy your snake. You'll find a lot of good advise on this forum, and I hope you become a regular contributor. Speaking of which, here's another great link (
Guide to posting pictures.) that'll help us to determine what species you have
. Kind regards,
Rick