API Algaefix is rubbish. I had experimented with almost all types of algaecides in planted aquariums. Copper sulfate is the the most potent but the therapeutic window is too narrow to be practical (it's more likely to be toxic than an effective treatment). Hydrogen peroxide is the best algaecide for a lot of reasons except perhaps in cases of green water algae where UV filtration is more effective. I had settled on a maximum dosing of 1ml of 3% hydrogen peroxide per liter of water for nuking algae, specifically blue-green algae. I had no issues with fish, shrimps, snails and most plants at that dose. Some sensitive and simple plants like mosses and Vallisneria can wilt though. I agree with your strategy of removing dead material following treatment as the nutrients in decaying matter will leach back into the water and possibly start another algae bloom. I do a massive water change with siphoning around 24 hours after treatment but that may be impractical to do for a pond.