It seems to me like not such stong growth for those shiners and you may have hit it with thinking about their feed.

I don't know how much time you have left this year for fish to take feed, but if your water temps are still above 70s or so, you might get some bang.

I'd get some small size fish pellets and throw some out for a few days and see what happens.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."