Solution for pasty but is to add 3/4 tsp of apple cider vinegar for a quart of water. You can add it to the communal waterer, it won’t hurt if other chicks don’t have pasty butt. If it doesn’t work, you can add more apple cider vinegar, up to 1 tsp. I recommend you try this electrolyte mix to prevent pasty butt in the future:
per 1 quart water
4 cups warm water
2tbsp + 1 tsp white granulated sugar or molasses
1/2 tsp salt
1/2 tsp baking soda
1/2 tsp apple cider vinegar
OPTIONAL: 1/2 tsp potassium chloride
Only feed this to chicks while they are in the brooder. Once they move out, it’s best to move to a store bought electrolyte mix. Just add this to their water. The best way to find potassium chloride is to find a salt substitute in the store. Just adjust the recipe to the size of your waterer. Hope this helps!!!
If you use your own electrolyte mix, I would add just the apple cider vinegar. You can also try mixing their feed into some scrambled eggs, plain, green yogurt or cornmeal.