The waterways get "algae blooms" where all the extra nutrients from the fertilizer helps algae grow rapidly. However, this algae all dies fairly quickly and becomes food for a lot of microbes which breath oxygen just like us, releasing carbon dioxide. So those herbivores/decomposers end up using all the oxygen and the fish die as a result. So it's not the plants/algae that deplete the oxygen, it's the herbivores and decomposers that can grow rapidly from having a huge food supply.