Copper sulfate root killer *IS* an effective algaecide; it is NOT a sanitizer. But if you maintain high levels of copper (2 ppm CuSO4 or 0.5 ppm Cu), you will prevent almost all algae. You will ALSO stain concrete pools and turn light colored hair greenish. You know your pool and your swimmers: if that's a problem for you, don't use it.
If you use copper, you have to keep the pH lowish -- 7.0 - 7.4 and NEVER shock. Shocking high copper pools can precipitate black copper oxide, instead of greenish blue copper carbonate or copper chloride, and will stain even vinyl pools.
It's possible though. Maintain low pH, low chlorine, and low stabilizer (7.0 - 7.4; 0.5 - 2 ppm, 20 - 40 ppm). We don't recommend it. It's almost as cheap to follow the BBB method (chlorine + stabilizer + borax) and not nearly so 'twitchy'.
Either way, you need accurate testing. People do operate like your sister-in-law without it, and a portion of them get lucky and avoid problems. But the only way to avoid problems, without depending on random luck is to test accurately and manage your chemistry. It's not that hard, but there is a learning curve. We can help with that, if you're willing.
Check out the test kit page, linked in my signature. You'll need a K2006 (A or C model) and also a copper kit, if you want to run copper.
Good luck!
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