Rookie pool owner -- small IG pool with SWCG

[jshelley64]

I recently purchased a home with a 3 year old pool. I have recurring algae issues and am not sure where to go. The pool guys keep having me add shock and phos prevent. When the water is tested it has shown a 0 chlorine level. My Pool Pilot Digital Nano has the check system light lit (solid red, not flashing) and there is no error message displayed. We cleaned the filter and they visually checked the cell and saw no issue. Any thoughts?
Pool Pilot Digital Nano model#75041
Pool Capacity 5500 gals
Concrete
Salt is showing as 3200

P.S. It was crystal clear when I moved in.

[PoolDoc]

1. Buy 5 gallons of PLAIN 8% household bleach, a gallon of DISTILLED water, and a cheapo OTO/phenol red testkit

2. Test your pH and chlorine levels tonight. Then add 1 full gallon of bleach.

3. Test your chlorine level in the early AM (it should be off the scale) and again in the late PM. Post your results here.

4. Order a K2006 testkit + AquaChek salt strips + a Taylor phosphate kit. Be sure to watch the videos, especially on the CYA test. Ignore the booklet and the 'wheel' calculator.

5. Keep us posted. Your results from #3 will give me some idea what's going on.

6. Complete the pool equipment form, so we have a clear picture of what sort of pool you have: http://pool9.net/pf-equip-form

. . . membership upgraded.