if you are feeding it for minerals I would feed a teaspoon per chicken per week.
But maybe fish would be cheaper for you?
My chickens love fish, the guts, (unless your fish have worms/parasites, in which case cook them first), the heads, everything. Sometimes the Salmon skin is too thick for them to eat, but usually they can eat it all.
I think that seaweed that you collect from the beach would be good too.
My chickens love everything.
Sometimes you can get slightly spoiled food from the markets for free or almost free. As long as the food is not moldy it is OK. I do actually feed my chickens food with a little mold.