sorry it took so long to reply...
so most of you agree with me, and it seems we are right, because today it's more obvious, the fin has become sharper, and his colours even brighter.
About the size, as they are my first batch, I don't know if they grow faster than normal. It's also hard to see how big they are, the picture is zoomed quite a bit.
But I have to admit that I was shocked about how fast these little fishies have grown in three weeks. I also have platy fry about 2 weeks old, and they aren't half as big.
Now I found one really special one (I think), a guppy fry that's one week old, and he has a bright yellow tail already
(or is this normal)
and umm, ftx, unfortunately I don't have a secret, except maybe a wonderful mommy and daddy guppy...