Stores that do marine water testing

I know that Petco does since they have saltwater tanks (and some local stores I know require water samples before they even will sell you any marine animals). I believe it’s free at any store I’ve been too, I’m not too sure of many that charge since they would love to upsell you later dependent on how your results came back.

I know my local store that is a family business does testing for both saltwater and freshwater tanks. No idea on brackish though, I’d assume so?

Big chain wise I think only Petco does

Petco is the only one I’ve heard of that does this, but I am also curious if there are other options out there.

There used to be so many mom and pop fish stores when I was growing up. They offered a lot of services for free but I don't think there are many around anymore. I'd love to see a long list of those types of stores around the US. But getting back to the OP, I'm not sure much is offered for free any more.

a bunch of local shops do it, especially before you buy fish from them. Maybe see if they can do it for free or a small charge while you get your tank up and running?