Pond Clear Algae Inhibitor Please note there is two products on this page. Natural barley straw will help keep your pond algae-free. Barley straw is completely safe for fish and can last up to 6 months. The bag option, there are 2 bags in each pack, each bag can treat a pond of up to 10mtr sq. The tub treats up to 2,000 gallons of water.  You can re-use the tub as an aquatic planter.