Diagnosing and treating as coccidiosis could help and that might be causing it, but I would say more likely that it's the brooder conditions. Coccidiosis is exacerbated by stress in chicks. If they are given proper conditions, cocci is exponentially less likely to show up. If you're keeping the waterers clean and the bedding is clean and dry (which it looks like it is) then it's probably not coccidiosis.