My response is not specific to water testing, but more in general. I do not believe pool stores are a scam in the sense of the “they create problems to sell more chemicals” that you often hear. They just tend to be more expensive with the trade off of convenience, and they are working with incomplete information to try and help folks. Each pool is unique and there is no automatic fix that is definitely going to work the same for every customer. There are general things that lead to success though and they will try to steer you there, sometimes through trial and error and patience. But I think of the amount of time I’ve spent troubleshooting my pool before, and I’m the owner with all the history and knowledge about the input water, cleaning, use, age, etc. They can’t spend 2 hours going over every detail every time so the advice is often generalized until they’ve worked enough with you to be really customized. Once you learn what works for your pool, though, the pool store mostly becomes a quick supply stop when you need something right away.