Rosy minnow having eating troubles?

For some reason the question won't post with all my details. :'(

I have 5 rosy red minnows in a 10gal, since about November. The tank is cycled, I do 50+% weekly water changes, and I know the tank is a little small but it's all I have right now (they were unwantedly given to me). Hoping to add to a larger community tank in a few months.

One minnow has recently lost weight. Upon observation, he's only eating flakes that are crumbled enough to fit right in his mouth, but before he used to have no problems tearing the larger flakes apart like his tank mates.

It's almost likes like 1) he can't see/aim for his food properly 2) he's being picky or 3) he simply can't eat the larger food. But I don't understand why larger flakes would become a problem only now.

I see no physical or behavioral signs of illness (no fuzz, no lethargy, etc.) he's just skinny. Nothing new has been introduced to the tank and everyone else is pudgy and healthy, so I don't feel like it would be a parasite either. So what's wrong? I can't keep overfeeding to insure he has food.

.....And it didn't even post in the right category. I'm so sorry people!