I'm not an expert on turtle nutrition, by any means, and I found this by accident, but...
Wardley makes a product called Goldfish Crumbles that seems like it has the right 'stuff' in it for turtles, but I'll ask the ones that have more experience.
Crude Protein - Min 35%
Crude Fat - Min 4%
Crude Fiber - Max 4%
Moisture - Max 9%
Phosphorous - Min 1%
Vit. A - 7600 IU/lb
Vit D3 - Min 600 IU/lb
Vit E - Min 100 IU/lb
Ascorbic Acid - Min 1400 mg/kg
Ground Corn, Fish Meal, Soy Flour, Corn Gluten Meal, Wheat Germ Meal, Dehydrated Alfalfa Meal, Brewers Dried Yeast, Ground Whole Wheat, Shrimp Meal, Fish Oil, Natural Flavor Fish Protein Concentrate, L-Ascorbyl-2-Polyphosphate, Vitamin Premix Containing (Wheat Middlings, Vitamin A supp., Riboflavin supp., Niacin Supp., Calcium Pantothenate, Menadione Sodium Bisulfite Complex, Folic Acid, Pyridoxine HCI, Thiamine Mononitrate, d-Biotin), Cloline Chloride, Betaine, Marigold Petal Extract, Ethoxyquin (preservative).
Is there anything in this that they can't eat? The D3 and Vit A are what caught my eye about it. I realize there's not enough calcium in it, but calcium is easy to come by in other ways.
Opinions, please?
Thanks in advance,
Mom and son, exploring the Herp world! 

