Background Reading: 6-G: Sanitation
Bacteria, slime and mildew can develop in neglected water, and grease and gunk can cling to the surfaces. The shell can be wiped clean but the inside walls of the pipes and hoses and behind the jets cannot. If there’s lots of jets, there theoretically could be more surface area in the inside walls of the pipes than on the visible shell.
Certain stubborn kinds of slime/mildew attach themselves to the surfaces, they can difficult to eliminate. Since the hidden plumbing and inside of the jets cannot be scrubbed, the algae problem could persist even if you empty and refill it with new water.
–Ask when was the water last changed?
–run your fingers along underwater horizontal surfaces. Does it feel slimy?
-any visible evidence of algea/slime?
-Check the chemical levels with test strips (bring your own fresh set). If the water is clear but chemicals are way off, it could be that the seller simply dumped the old water, wiped down the shell and refilled it with fresh water.
-Pop out one of the jets, check from the back for crud, and smear the jet body with your finger. Is it clean?