My two garters, but some say they are actually ribbon, eat salmon, trout, atherina boyeri (small fish F/T or fresh). Refused pinkies, whole or parts, refused guppies. Not yet offered nightcrawlers due to unavailability in my country atm.
My old couple, who were surely sirtalis sirtalis, ate all the above save pinkies, plus veal and beef meat stripes, especially heart, plus any kind of worms (most probably they got some red wrigglers too), plus toads, frogs and tadpoles (the latter three, captured in the lake). Interesting to say, thy never ever got problems, of any kind.