Cut zucchini lengthwise into quarters. Coat with small amount of oil and add salt and freshly ground black pepper to taste.
Season tuna with salt and pepper.
Place zucchini and tuna on the grill. Grill zucchini about 2 minutes per side. Grill tuna for about 5 minutes per side or until it is no longer pink. (Sushi grade tuna may be seared on all sides but served with a pink center.)
In order to calculate Net Carbs of any whole food item, it's a simple calculation based off the Nutrition Facts panel:
Total Carbohydrates - (Minus) Dietary Fiber = (Equals) Net Carbs
For low carb products sweetened with sugar alcohols (ie: glycerin, maltitol, etc), refer to the Nutrition Facts panel of the product:
Total Carbohydrates - (Minus) Dietary Fiber - (Minus) Sugar alcohol = Net carbs
This reflects the fact that Dietary Fiber doesn't impact blood sugar levels. Sugar alcohols also have a negligible impact on blood sugar, they too are subtracted from total carbs to yield the Net Carb count.