Due to the sudden onset of syptoms, I agree that the goldfish were probably treated with something toxic. The medications administered to fish to treat problems like ick, fungus and bacterial infections can make them unsafe to use as feeders. This would apply to any type of petstore fish.
Goldfish also have an enzyme that breaks down the B vitamin thymiamine. When fed as a dietary staple, long term, this can result in nutritional deficiencies. This makes fish like guppies or minnows a better choice.
What species of garter do you have? Most can be converted to rodents fairly easily if you're willing to put in a little work. Rodents have the advantage of typically being parasite free and nutritionally balanced. It's best to select a young snake as they tend to be more flexible.
There are a few methods that work to get them onto rodents. First try an appropriately sized rodent straight up; sometimes you get lucky (you can use a long pair of forceps to mimic movement if feeding pre-killed). If it doesn't accept the mouse offer it fish instead to get it eating. The next time you feed put a washed pre-killed mouse in the bowl with a fish. Gradually you can stop adding fish and just put a mouse in the bowl, and eventually just a mouse in the cage. If this doesn't work you can try rubbing a fish on a washed pre-killed mouse and offer it on forceps or put it somewhere the snake will run into it. If this still doesn't work, you can take a dead fish, cut off a piece and put it on a pre-killed washed mouse. You can gradually decrease how much fish you put on down to none. If feeding anything larger than a small pinky, it's best to feed frozen thawed or pre-killed rodents for the safety of your snake.
I have four garters including a western black-neck I caught in Arizona a couple of years ago. I got them all to take rodents using the above methods. The western blackneck was the most difficult, but I got it started using the piece of fish on a pinky method. They seem to be able to smell the mice as soon as I walk in the room on feeding day and start racing around their tanks in anticipation, and grab the mice off the forceps as soon as I put them in the tank. I've heard that the AZ checkers and wanderings are typically easy to convert.
-Alice