Can't keep chlorine

[Stumped]

I'm giving up and closing the pool this weekend. I have had the same problem all year of not being able to keep chlorine in the pool. I can double shock the pool at night and by morning it's gone. I had my water checked and everything was fine. I finally decided to test for nitrates and the reading was in the 10-20ppm range. I heard if you get a reading at all its too much so I drained half the pool water and refilled it last weekend. Afterwards I tested for nitrates again and got the exact same reading. And yes I tested my tap water which showed zero nitrates. Since then I still have the same problem. I shock the pool and its gone in hours. Here are my latest test results for my 17,000 gallon above ground pool:
FC- 0.5
TAC- 0.5
PH- 7.4
TA- 70
Calcium Hardness- 120
CYA- 35
Total Dissolved Solids- 600
Phosphates- 0

My water is clear but every couple days I can see algae starting to grow again because I can't keep the chlorine up. I bought some product to increase the hardness. Should I increase the TA? What do you recommend when I close the pool? Should I bombard it with bleach and hope for the best? Thanks and good riddance to this season!





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[pool kid]

if you have a reading of 10-15 ppm of nitrates what i have done with my customers who have that reading is shock the pool ten times the amount of chlorine. So say one bag of shock does 10,000 i would say for your pool you would have to do shock two bags for a normal shock.. but with a reading of nitrates you would have to do 20 bags of shock. now, normally thats an expensive proposition. it is alot cheaper to get a big five gallon of liquid chlorine. Since one of the five gallons of liquid shock equals 7 bags of shock i would recommend going with the overdose and doing 3 five gallon liquids. I know this sounds ridiculous but at my pool store i get a lot of customers who live near woods and they always get nitrates and i try this method. all of them come back with great success.. their pool clears up and there chlorine stays stabilized... if you closed it already. i recommend when you open up the pool throw whatever bags of shock you have and let it run for two days... then hit it with the 3 five gallon liquid shock.. try it and let me know

[chem geek]

As I posted elsewhere, shocking with chlorine does NOT get rid of nitrates nor phosphates, but it will kill an impending algae bloom. However, if you don't then maintain a high enough Free Chlorine (FC) level of 7.5% or more relative to the Cyanuric Acid (CYA) level, then the algae can grow faster than the chlorine can kill it.

Many people shock their pools with chlorine only to have algae return if they don't subsequently maintain high enough FC relative to CYA.