We feed ours this roughly
Morning:
Cucumber -1.5 cm x 2cm chunks (twice)
Celery - 1 stick between 3 chopped up small
1/6th of Red/Green/Yellow Bell Pepper (de-seeded)
Evening:
Cucumber 1.5 cm x 2cm chunks (twice)
Two Little Gem Lettuce leaves or 10 sprigs Coriander. Twice a week we will swap in cabbage or parsley
We also add in to the above diet carrot, sweet potato, celery leaves and a few other things occasionally
But by no means am I any expert on food here, some folks on the forum are really clued up... so pick up advise where you can.