Barley straw cleans ponds and it’s safe for fish.It’s also safe for livestock. We use barley straw bags in our horse tanks. As long as you change them regularly (see the label on yours for info), they work great!
you could add some fish to your water reclamation tank for a small aquaponics system. Grow a few plants to feed the fish and this forms a closed system. There is no need to fertilize, the fish do it for you! Permaculture at its finest!