Persistent algae

[wdwfan]

I have been having alot of trouble with algae this year. One week ago I finally had the pool all blue with no signs of algae & all the levels correct, except slightly high chlorine. Then, we went away for 2 days & when we came back it was turquoise. Because the chlorine was high I put in a non-chlorine shock (Tues night). Thursday night I was able to vacuum stuff from the bottom, but some kicked up & turned it turquoise again. The next day it was a darker turquoise & it looks like the algae is floating again (I can see the bottom of the pool, though). Here are my details:

18x33 above ground. This site calc's that to be 53,000 litres (or 11,600 gallons/14,000 US gallons)...I'm not sure which works for you: I'm in Canada so we go by litres.
Sand filter, no heater.
Test results today from pool store:
Free Chlorine 5.4
Total Chlorine 6.3
Combined Chlorine 0.9
pH 7.0
Hardness 200
Alkalinity 105
Cyanuric Acid 83
Copper 0
Iron 0
Total Dissolved Solids 300

I know my pH is slightly low: every year I have had to add pH up on a regular basis, but this is the first year I've had consistent algae.

Verbally, the pool store only recommended that I had c-clear (which is a floc product I believe). On the sheet they gave me it mentions increasing the pH, superchlorinating (although my chlorine is already high?) and then adding a "Potassium Monopersulfate"??? Is this floc?

Thanks for any help. It's going up to 40 celcius with humidex tomorrow if that makes any difference.