If they have access to ground, and you don't have them on medicated feed, I would put the Corid in the water. Cocci can kill a chick quickly and legarthy is usually the first sign. Sometimes the medicated feed is ineffective and occassional treatments with Corid in their water is necessary...
Are they in a brooder or on the ground somewhere? Have you added anything to the water? It's hard to determine if a chicks crop is emptying because you don't want to remove their food in the evening, but I guess you can put them to bed in the evening and check early in the morning before you...
What kind of bedding is the chick on? Shavings? Fine shavings or big flakes? You want big flakes, not the fine shavings which they tend to eat and can cause blocked crops and gizzards. Don't want hay either. That's just as bad.
Crop does look big. Did it go down in the morning? If it...