If you identified which chickens are doing the pecking you can either separate them, rehome them, or try the pinless peepers. Pinless Peeper look like funky sunglasses that fit on your chickens's head. These plastic blinders prevent the aggressive birds from seeing what's in front of them so they can't peck the others.
You have 20 chickens in a 24 x 20 coop? Might want to cut that number in half, add more protein to their diet (cheap dry cat food will do fine), and give them other things to peck at.
One of my favorite (and cheap!) chicken toy is to take an empty 16 oz plastic bottle (can be smaller or larger) put several small holes in the side, fill with scratch feed, screw cap on and leave on the floor of the coop. The chickens will learn to move the bottle by pecking it to make the scratch feed fall out. You can hang the bottle if you want to. You will have to make sure the feed is clean and dry in the bottle. Make several so you can clean and dry a few bottles while other bottles are being used.
You can add other greens to their diet: Kale, Kudzu, Clover, Spinach-20 chickens can eat a 5 gallon bucket full a day of the fresh stuff. You can feed them cooked beans, carrots, peas, corn on the cob. Table food won't go to waste if you have a flock of chickens. Don't be afraid to use your imagination on how to keep your chickens entertained. They'll appreciate the challenge and you'll have fun watching them.