Rowaphos is a form of GFO (Granular Ferric Oxide), it is not a pesticide, it is a water purifier.

Versions of GFO are actually used extensively all over the world by water companies to remove arsenic, phosphate and heavy metals and purify mains drinking water that everyone drinks every day.

In an aquarium Its function is to chemically adsorb PHOSPHATE from the water. The phosphate is produced by the breakdown of fish waste in the aquarium and the Rowaphos purifies the water by bonding the phosphate to itself so that it is no longer free in the water. If the phosphate was allowed to build up, then it would start to have harmful effects on the fish and corals in the aquarium.

Plants and algae use phosphate as a food, it is a fertiliser, and this makes them grow. 

By purifying the water with Rowaphos and by preventing the levels of phosphate, created by fish waste, from increasing to excessive levels, it prevents algae in the aquarium from using the phosphate as a food and explosively growing. Such algae growth can take over the aquarium and start to kill the corals and fish over time.

Unlike a pesticide, (herbicides, fungicides and insecticides), which are normally toxins designed to kill pests, Rowaphos purifies the water so that nuisance algae cannot use the food to grow in the first place.

An analogy would be if you had a tank of water outside in the sun. Like phosphate, sunlight acts as a food for plants and algae, so very quickly the water would go green. If you shaded the tank with some sort of ‘cover’ then the sunlight would not get to the water and so the algae would not be fed and would not grow. However, you would not call the ‘cover’ a pesticide just because it prevents the growth of algae. 

Rowaphos works in the same way by taking away the food so that the algae does not grow. 

A further example, water plants absorb phosphate and so large numbers of such plants in a pond or aquarium will lock up the phosphate in the same way as Rowaphos does and prevent it being available for algae growth explosions.

You would not call a plant a pesticide.

Rowaphos is a water purifier.