Garlic can be toxic to chickens if too much is given. I would get some Corid, and give them a 5-7 day treatment of that. Also get some SafeGuard Liquid Goat Wormer and treat each chicken with 1/4 ml per pound of weight for 5 consecutive days.
How old are your chickens? Have you tried to get your regular dog or cat vet let you bring in some droppings for a fecal float as a favor? That would tell you if there are worms or coccidia. Worms can cause blood in droppings. There can be resistant strains of coccidia, which might need another...