Im surprised that you need anywhere near as much as that thiosulphate to neutralise fresh water. I use it at work if I'm trying to sort out a customer's swimming pool/spa that they've overdosed with chlorine and rarely would use that much, even on much bigger swimming pools with free chlorine readings over 10ppm. I would have thought 100g would be more than enough, and not have as much of a negative effect on the pH levels
Well that was the output from a koi site calculator. I have two 500g bags so I don’t need to use it all and I can test the chlorine level. Koi aren’t sensitive to it plus it’s used for Potassium permanganate overdoses should I get a measurement wrong.