algae problems in spite of 3 ppm chlorine

2nd year pool owner...

Having problems with recurring algae this year. Water fairly clear, but green algae continues to build up on floor and walls every 4-5 days.

Latest example...
Friday - green powdery alage on pool floor and walls. Not really visable unless brushing. shocked pool to 15ppm, kept there thru Saturday.
Sunday am vacuumed dead algae (white/grayish). No powdery green algae.
Monday - water getting clearer, but green powdery algae back on floor/walls. Free chlorine down to 8.5. Vacuumed again to remove algae.
Tuesday am - water very clear, but green powdery algae back on floor/walls. Free chlorine down to 6.0.

Levels maintained very consistently
PH = 7.4
Free Chl = 3-7
Alk = 110
CYA = 70

Have been using 3-4 PPM of Chlorine per day, which seems very high. Pool in 100% sunlight. backwashing regularly to clear after each cleaning.

Do I simply need to keep shock levels for a longer duration than two days?

20,000 gallon
Vinyl liner
Sand filter

What are you using to shock with meaning ingredients and not product names.

With a CYA of 70, you have to take the chlorine up to 20ppm to be at shock level.

What kind of kit are you using to test the water?

Please list all products (including ingredients) that you have used.

Welcome to the Pool Forum!

Using Tri-chloro 3" tabs for daily chlorine.
Using Calcium Hypochlorite to shock and to raise Chlorine levels quickly when needed.
Using HTC super algae guard (Alkyl Dimethyl Benzyl & Ethylbenzyl Ammonium Chlorides)
Periodically drain 3"-4" and refill to keep CYA and calcium levels in check.

Using a Taylor K-2006.

Current levels...levels maintained very consistently
PH = 7.4
Free Chl = 5.5
Alk = 90
CYA = 70

Pool was very easy to maintain 1st two months of use. Have been fighting green algae on & off 2nd half of last summer & this summer. Took CYA down to 50-60 at beginning of season, now up to 70.

20,000 gallon
Vinyl liner
Sand filter
Run pump 24x7

1. Stop using the "Algae Guard"; it's not helping, and it is using up chlorine.

2. Read the Best Guess page linked in my signature block -- it will explain what chlorine levels you need, and why.

3. If you want to make an investment in making your pool algae resistant, you can do this at the same time you are working on the controlling the algae. 60 ppm of borates seem to make a pool significantly more algae resistant, though it's not clear why. In your case, that means about around (20) boxes of borax and 5 or 6 gallons of 31% muriatic acid. The most conservative way to do it is to add 4 boxes of borax -- slowly through the skimmer with the pump running -- then 1 gallon of muriatic acid to the pool, and then circulate the pool for 4 hours, test, and adjust pH to 7.2 - 7.8, and repeat. Slow down on the last 4 boxes of borax, so you can dial in your pH. You may end up with a bit of borax OR acid left over.

4. Get a cheap OTO/phenol red (yellow / red drops) and use it side by side with the K2006, so you can get the hang of recognizing the OTO color that corresponds to an adequate chlorine level in your pool with CYA=70 ppm. That way, you won't use up the test reagents in the K2006 so fast.