Also. The little daphnia monsters, compete with the hydra for any available bacteria, in addition to eating green algea. Fry and small fish prey on the daphnia, were nothing will eat hydra, that I know of.
If you have a infestation of ick, hydra, planaria, worms, or other sump monsters. An algea scrubber is a priceless tool. Possibly worth more than, all your bio media, and a kilo of Purigen.
Some want nitrate reduction, I want a battle scarred, fin nipped African Cichlid to heal him self. With out any pharmaceutical products.