@j.w scroll down just a bit further on the amazon link you posted. I bought the 10 gallon bucket that says it's 100% pure.
I've bought it for years from a soap making supplier - it is the exact same product used in laundry products, but they add more things that I for SURE wouldn't put in my pond.
SPC breaks down into hydrogen peroxide and soda ash - which is frequently used in toothpaste. Harmless. And like
@WaterGardener said, it's way easier to use than liquid H2O2 because it's a powder and you can apply it right where you want it. I've been using this in my pond for years with zero ill effect. My pH stays stable, my fish stay healthy and my pond gets clean.
Look up Aquascape EcoBlast or GreenX, among lots of other pond related products. Many places sell it as a "granular algaecide" which it also is - it's great for cleaning off waterfalls and rocks in the spring. Takes care of string algae in a single application. I first discovered this product when I bought Aquascape's SAB (String Algae Blaster) which was 100% SPC at that time - they've changed the formula now, but that was my "AH HA" moment. I paid 20 bucks for 8 oz of a product that was dirt cheap when I bought it in bulk. Bought myself a 50 gallon bucket at that time and never looked back.