Hi. New here with a question.

[Stefan-A]

The shop recommended feeding it on fish, saying that they had been feeding him bits of cooked fish, even bits of Fillet of fish burgers!!!(this cant be good?).

Yeah, it's not. Some shops...

Now my main question is: with my other snakes; Corn and Childrens Python, to feed them I take them out of their Vivs and feed them in a serprate container, to avoid any ingestion of substrate, (and also cos I like to give them a good check over).
Should I do this with the Garter snake, or feed him in his container?

I recommend it, if he'll eat after being moved to the other container.