Pure Nature Pets Poseidon Filter Rocks, 5-Pound Poseidon filter rocks and made from ammonia capturing zeolite to reduce ammonia levels in aquariums and ponds Enhances the startup and helps maintain biological filtration by providing a growth medium 100% natural Clinoptilolite zeolite Organic & all-natural; processed and produced in the USA 2" - 4" in size, roughly 25 pieces per package