I recently hatched my first ever S. santanae from Jim Kostich. It was a huge popcorn-like hatch which was really exciting to see. My question is about a observation I have had about the developing fry. After about a week in their post-hatch container, I moved the fry (about 30) to a 5 gallon grow out tank. After a couple days in the grow-out I noticed that about 1/3 of the fry were lighter in color and tended to stay in the middle of the water column, while the remaining fry were darker colored and tended to be at the bottom of the tank. Not unwell; they swim just fine and are active. Now at three weeks post-hatch, the differences remain. It is also clear that the fish have different morphologies: the midwater fish are not only lighter in color but are deeper bodied, sort of robust little pupfish; the bottom fish, while at least as long, are darker in color and have a more stream-lined, gracile profile. Again, they appear quite well and swim up in the water column with ease when they want to.
Being new to this species (and genus) I was just wondering if anyone else has had similar observations? Perhaps it's as simple as sexual differences, but I have not seen anything quite pronounced as this at such an early stage in any fish I have bred. Any thoughts?

Hi Phil.  I have noticed this in a couple of batches of santanae fry as well.  I have always presumed that it was due to some of the eggs hatching either a little prematurely or more likely postmaturely (if there is such a word?).  After a few weeks, the differences seem to disappear.