If you have algae again, it's because you either 1) let the chlorine get too low, or 2) never killed it off completely before allowing your chlorine to come back down. It has nothing to do with your pH. Your pH is fine where it is--you just need to raise your chlorine back up to shock level (best to do that with bleach instead of your SWCG). How high shock level is depends on your current CYA level. Do you have the test kit yet so you can test your CYA?