Feeding fish is a lot of fun, be sure to buy clean Rosie reds or guppies from a store with clean tanks and no dead ones floating around in it.
Some garters will eagerly take the fish from your fingers, just dangle it in font of the snake.
And fish are sticky, so the best 2 options are either to feed in a separate area (we use tote boxes) or "Cage carpet" (I like this stuff) with a "Cage Carpet" you can just toss in fishes and earthworms no problem. 
Sometimes the snakes will grab the fish and strut around with it for a few moments before eating it. Very entertaining to watch.
Ditto on the goldfish warning, no no not good. Goldfish are toxic.
We set up an aquarium and buy many rosies at once, but then your buying fish food, and changing filters, etc.. so that may be extreme.
Please take a picture of your snake out of the tank, it looks like a very handsome animal, and pictures are always cool.
Kind regards,
Wayne A. Harvey

our critters
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1.0.0 Ball Python - Python regius "Cane" Rescue
0.0.1 Eastern MilkSnake WC "Carmello" adult super sweet temperment
1.2.3 Storeria dekayi Casper, Xena, Athena, & Kids (30 plus released!)
0.0.2 Thamnophis marcianus - Checkered Garters "spot" & "Zig"
2.2.0 Thamnophis radix Snow, Iowa snow & christmas snow het.
2.1.0 Thamnophis sirtalis - eastern Garters
0.0.1 Thamnophis sirtalis - "Flame" red and white stripes
1.1.0 Thamnophis sirtalis CB "Flame" & Melanistic
1.0.0 Thamnophis pickeringi - Puget Sound Garter "Sky" (adult, Sky blue)
0.0.1 Nerodia sipedon - Water Snake - "Aqua" adult WC
0.1.0 Storeria occipitomaculata - Red Belly snake (Her children were raised & released)
0.0.1 Amelanistic Corn Snake "CY" Juvinile CB
1.0.0. Pueblan Milk snake "Oreo" adult CB
1.0.0. ASIAN GREEN SNAKE 3' WC Cyclophiops major"Limon"
0.0.1. Savannah Monitor "CHOMPER" Growing fast!
0.1.0. Green Anole "Crystal" WC
1.0.0. K9 "ACE" Black Cockapoo
0.2.0. Feline"Felix"(R.I.P. 4/27/08) "Kaja" & "Silver"
2.1.0. calico RATS
2.4.?? Mice - Feeder farm - Crickets / fish
More herps than I could ever list out back on the land. 
