Looking through the catalog "the pond guy" sent me... Mechanical pond maintenance is 10x more expensive that chemical. I can buy $100 bottle of duckweed killing chemical that will last for several years. Just an aerator for the same size pond costs $2500 not including installation and electricity.

This is the same reason why pasture raised beef, lamb, or poultry is more expensive than industrial farmed meat. Chemicals to prevent worms are cheap, but labor to move these animals frequently is expensive. But one has negative externalities and the other doesn't.