Hatsoff,

You might have come to the wrong place: you think test strips are great; you trust your pool store; you are using HEDP (Conquest) at their instructions to remove an unidentified gray stain. It sounds like you've got a plan and are pursuing it.

It's not a plan we'd recommend, but it's your plan and that's what matters.

What's not clear is what you want from us?

Our advice is not really compatible with pool store advice, so you'll end up having to pick one or the other. It sounds like you just want a 2nd opinion, but I'm pretty sure you don't want OUR advise which is going to going to include things like, "Ditch the 'guess-strips'" and "Figure out what the stain is, before you start treating it" . . . and you've already decided you don't WANT that sort of advice.

So, my first thought is that we can save both your time and ours, by suggesting you go with what you trust, and head back to your pool store!

We hope you have a great pool season!