If you decide to use that CYA instead of trichlor or dichlor to introduce stabilizer, I wouldn't add enough to add 50ppm right off the bat. Your test showing 0ppm, simply means that you have less than can be measured so that doesn't necessarily mean 0. You have to have at least 30ppm of CYA for the test to detect it. So, you may have some CYA in there -- maybe almost 30 so if you shoot for adding 50, then you'll end up with more than you want. I'd aim for adding 20 and then a week later, measure it and see where you are. You can always add more later if you are not where you want it to be.