Fresh food (veggies/fruit) for Comet Goldfish a good idea?

I regularly feed my 2 large 3-year-old comet goldfish their usual
pellets, and they appear healthy and happy in their new 33 gallon
tank. However--despite the fact these pellets claim to have
everything the fish need--I'm thinking of throwing in fresh food, such
as finely chopped mango, strawberry, etc., maybe some veggies (soft
ones, of course) to vary their diet

Is this a good idea? Are there any fruits or vegetables that they
shouldn't eat? Do you think it would be better for their diet to
receive such stuff?

Thanks,
Adrian
Victoria, BC, Canada