Canned fish almost always contains a significant amount of salt. It might as well be poison. Garter snake's bodies do not need salt at all and they do not have a way to expel excess salt like mammals do. Marine iguanas are about the only reptile that can handle salt. Other than the salt or other additives, there's nothing wrong with canned fish even though it's cooked but cat food is a horrible thing to give your snakes.

It may seem expensive when you see the price per pound for safe fish fillets(trout, salmon) at the supermarket but I can feed 6 huge adult concinnus and about 50 babies for around $7 a month so it's not that bad.