Your best bet is to get water lettuce,duckweed,giant duckweed, all that floating pond stuff, most people will give it away due to its rapid growth rate.
I fed mine minnows once a week, I put 18 of them in there to last a week(had filter and mud bottom) but the rest of the time I would offer it the sticks and I always kept floating plants in there. You can go door to door on lots that have the ponds with the stuff in it, take it home rinse it well and put it in the tank too, thats where the free comes in and you could probably work something out where you can go back weekly for fresh greens.
I got mine as a rescue, 6" in a 10g tank, got her a 40B right away with heat,filter and lighting, but once she started eating the minnows and the greens she literally turned into a happy turtle and the colors were alot brighter and more distinct.
I miss my shelly but she moved to a pond with a RES named bob, she became a mom.
