No. Just add borax every 2 hours. If your pool is 20,000 gallons or more, you could go to a whole box.

You could get an acid demand test, but it's hard to use them accurately. The problem is, your pH will look like it's not changing at all, and then began to change suddenly.

Ben

PS. When you scraped the black algae, did you SMEAR it on a white card? If it doesn't smear green-ish, it's not algae.