We recommend to stock tadpoles and snails in the pond because they are scavengers. Their main food consists of algae, debris and detritus.
Thus, they assist the pond by reducing the amount of fish waste and decomposing plant matter either suspended in the water or piling up on the bottom of the pond. This is beneficial as the fish waste and decomposing plant matter are the two main sources of nitrates and phosphates; two of the main reasons for an algae bloom is an abundance of nitrates and phosphates. Filtration is also beneficial in reducing these two chemicals, but it is definitely a good idea to have as much assistance as possible in the form of tadpoles & snails.