It really depends on how "hard" the water is with that one. (The general rule is toxicity increases as water hardness decreases). However it is cheap and easy, so a lot of people still use it. It is very effective, although it is pretty nasty for fish. With the chelated CuSO4, there really hasn't been a lot that compares the effectiveness of both types (chelated or not) with both hard and soft water. I think there were a few studies that actually showed no difference, or rather an equal toxicity to fish.

Although I have a master's in fisheries, my main focus was a bacteria transmitted to humans from consuming raw oysters. So I am mainly a microbiologist by training. However, I did take quite a few pond management classes and still have some knowledge from them! I am not claiming to be an expert in pond management by any stretch of the imagination!