I guess my first answer didn't post. It is likely that they are goldfish. I have had only comets for five years. The babies are always grey or black and turn orange after a couple of months. I have two right now and one is just beginning to change color this week.

Five of my three inch long goldfish babies are still black. I also have inch long white and orange babies. Don't know why some stay black so much longer than others in the same environment. Must be genetic.

That explains why I never see tiny goldfish babies; there are lots of grayish mosquito fish in the pond, and the baby goldfish blend right in.

Now my babies have disappeared. Maybe mom and dad and uncles and aunts ate them! Sad

A miracle! I have a lotus pot shaped like a flower that I have a mini waterlily and 1 water hyacinth in and today I saw a baby fish. I guess he came in as an egg on the water hyacinth I pulled from the bigger pond. I can't wait to see what he turns into. I have goldies and shubunkins in the big pond and they usually eat all of their off spring.

koi babies are sometimes brown too.
I'd look for the "whiskers".
I hear that there are koi/goldfish mutts too.