Sorry ´bout the delay. I had some trouble over here with the comp. Any way, if you get a gartersnake, make sure you are calm and at ease with the fact that it is a `racer´. NEVER go about groping franticly for you pet, especially with garters. That will only arouse it even more. Stay calm and approach the snake slowly. Take into consideration that most dangers in the wild for a snake come from the sky, so try and make a low approach. An approach from above will needlessly stress out the snake. Take out the snake regularly and ´play´ with it. Have it do figure eights aroud your pinky and your thumb to calm it down. All smaller snakes absolutely LOVE crawling around your shoulders, into your hair and, if you´ll allow it to, into you garments.
I am no fan of underheaters, because the snake can sense too much heat from above, but not on its belly. Some poor critters have litterally fried their tummies on rock heaters. Place a nice branch of some kind or some rocks in the tank so the snake can climb atop, and bask. It will sort itself out. If it gets cold, it will seak heat, if it is too hot, it will seak shade. Make sure it can find both.
I use a 25 to 40 Watt bulb in my tank. It provides adequate warmth and, in combination with a large ovendish for a bath, it also keeps up the humidity. When it seems my pretty needs to shed, I switch the bulb for a 25 Watt Ultra violet lamp, so the extra UV light helps ease the shedding.
About the food: gupies are fine, but when Casey grew lager I started on small goldfish, because he would eat 3 guppies a feed and STILL go looking for more. Now he is on a 4cm goldfish a week.
Have fun with your pet!