Sodium tetraborate is borax.

You can use it -- doing so makes your pool less 'friendly' to algae -- but to reach an effective level, you need at least 30lbs more. You can buy it at Walmart as 20 Mule Team borax in 4.75 lbs boxes, so you'll need about 7 boxes. PLUS . . . borax is basic, with a pH a little above 9, so you'll need 4 - 5 gallons of muriatic acid.

Do NOT backwash, unless you see a pressure increase -- otherwise, you'll have a hard time removing the fine particles. When you do backwash, notice what color the backwash has -- if it's green, increase your chlorine levels a lot!

See if you can find an HTH 6-way kit at your local Walmart; if not please order a K2006. Test strip measurements of CYA are more often wrong than right -- and CYA level is a KEY factor in algae control.