They'll certainly enjoy the krill. In nature, a large part of their diet is vegetable matter and in the process of ingesting that, they take in a lot of small crustaceans and the like. The chitin in their shells are a good source of roughage for your goldfish. The only thing I would be concerned about is making sure that you balance the protein with an appropriate source of veg matter. Unsalted nori sheets are a good choice, as are many blanched leafy veggies.