Green or red leaf lettuce. You can blanch it for 30 in boiling water, but don't have to do this. Clip the lettuce to the side of the tank, and they will nibble on it. Might take a few days for them to figure out it's food--be patient, but make sure it doesn't start breaking down. Zucchini and (odd, I know) orange slices can be floated in the tank for them as well.