I had one hatchling from a marble spawn so for the first 10 days he was in a solo cup all by himself. Not one left...one TOTAL. His egg was the only one out of hundreds that hatched.
When he was 14 days old I put him in with my yellow spawn that is 2 weeks older than him. Hopefully you can see the color difference. Something interesting to note. The dark colored fry is just a bit over two weeks old and is larger and more fully formed than his older friends.
And here is one of him(could be her, lol)