Blue dye inside new(ish) tank

I recently got a 7ish-year old ball python, who came with a 10 gallon tank. I'm looking to rehome him in something larger, so I got a 15 gallon tank on OfferUp. I was washing it out today in preparation for moving him in, and I noticed that there's some blue staining/buildup on the inside of the tank around where a water line would be (guy I bought it from said it was used as a fish tank, but he didn't specify what kind of fish). I'm thinking it was a water dye of some sort or something left over from its fish days, but does anyone know if it's okay for snakes to be around? Some antibacterial dish soap and elbow grease got it off of the glass walls, but it's still staining the glue joints.
Thanks!

Most likely a treatment for ick which is a fish disease. Probably not enough left there to hurt the snake but you may want to look it up.

I've treated fish tanks with a few different medicines, including ick treatment, that have a blue or purple hue to them. I'm guessing that is what stained the glass. I'm pretty sure you'll be fine after giving it a good scrub, but like Jodan said, can't hurt to do more research before going ahead with using it.

An adult ball python will still be cramped in a 15-gal tank.

Since he needs an upgrade anyway I would suggest something like this, which is a lot more secure and holds humidity better:
https://www.lowes.com/pd/Hefty-100-Q...g-Lid/50150118

Add an under-tank heat pad and a thermostat, substrate, water bowl, and décor and you're set.