heater help

i'm new to this turtle thing and have been looking everywhere or help. any advice you can provide would help.

i am the foster mother to 2 red eared sliders. one is about the size of a post-it note, and the other about the size of my hand. i don't know a lot about them, as they were my mom's boss' kids. the kids outgrew them, and were just going to get rid of them when i said i'd adopt them. the people that had them had virtually no stuff for them...just a 10G tank, food, and a rock.

since i've had them i've been reading up a lot, and now they have a 30G tank, a filter (zoo med 501), a turtle dock, a basking light, etc.

i wanted to get a heater for their water since i live in a cold area and read that their water should be 78-80 degrees F. i had one for a fish tank, and it cracked. people have suggested those under tank heaters like you'd use for an iguana. does that heat hte water?

what can anyone recommend? i wish i knew of something specific for a low depth tank.

sorry this is so long. any help appreciated!

hello? is anyone out there?