+1 for white vinegar.
we try to buy as much used equipment, including tanks, as we can rather than purchase new. I always scrub with vinegar and an algae scrubber/natural sponge. Rinse really well and put into service.
If you’re on a budget, check out your local fish club classifieds, FB marketplace, Craigslist, etc. you can sometimes pick up a whole tank set up on the cheap. That’s how we got our 10 gallon quarantank plus accessories for $40.