And they swam and they swam

We have a problem with our goldfish. They seem healthy enough, and are swimming about in the pond quite happily, as far as I can tell. I mean, they’re not going around in a little fishy conga-line (or even conger-line. Sorry), blowing trumpets and letting off sub-aqueous party-poppers or anything like that. But neither are they obviously sulking and throwing tantrums (tantra?). There are about 40 of them, in a surprising range of sizes considering they’re all the same age.

But they're not gold. They are, with one notable exception, black. The pond liner is black too, with the result that I find myself feeding invisible fish. What is the point of ornamental fish that nobody can see? Is there anything that can be done to goldify them, or should I start looking for suitable carp recipes?