Chickens are omnivores and therefore will eat almost everything. Here is
What NOT to Feed Chickens: 9 Food Your Chickens Shouldn’t Eat.
I'll sum up the list for you, but the article goes more into depth why....
1. Raw green potatoes
2. Onions
3. Asparagus
4. Nightshade plants
5. Avocados
6. Dried or raw beans
7. Anything moldy
8. Dairy
9. Chocolate
If you think about it, that's a very small list. Almost everything in our kitchen as leftovers is fed to our chickens. I have gone to a deep litter composting chicken run and any scraps that do not get eaten by the chickens gets mixed into the litter and composts in place. The worms eat the decaying matter and the chickens will dig down and eat the worms.
Some people don't realize that chickens also eat meat. They do, and love it. I feed them all our leftover steak bones, chicken bones, etc... They will pick the bones clean and then the bones get mixed into the deep litter and compost in place.
When I go fishing, I will dump the fish remains after cleaning the fish into the chicken run. The chickens will peck the flesh and eat what they want. The rest gets turned into the deep litter and becomes worm food. Depending on how much fish remains I have, I use some common sense not to let smelly decaying fish remains stay in the chicken run attracting predators. If you don't use a deep litter composting system in your run, I would not leave such remains overnight.
Bringing the subject back to the original post, if I have more fish remains than my chickens can eat during the day, I will just dig down into my grass clippings in the chicken run and bury the fish remains. But as I said, my deep litter is about 16 inches deep now and is a mix of grass clippings, wood chips, and leaves.