Fiberglass hottub black algea or mold treatment?

Apr 24, 2017
1
Tampa
#1
Indoor small fiberglass hottub. Lots of black spots on tub surface. Drained tub, scrubbed with bleach. Filled tub, added bleach, circulated for an hour and drained again. Cleaned filter with hose. I still see vague bits of leftover black spots. Pool guy told me to let tub dry for a couple days. Now what do I do to keep it dead? Bleach more before refilling, or refill and treat with extra chemicals?
 
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