Great info on this site! Thanks to all contributors!
I have a pool that's about 18 months old. For the last 9 months, I have been at war with blue/green algae and probably some black algae as well. The algae normally resides on the pool walls and floors and has clouded the water on 3 separate occasions (blooms). Brushing removes most of it, but I do get noticeable green stains on the blue pool surface.
I have been through three different pool shops and been burnt by pool shop staff not know what they're talking about coupled with questionable results from their test equipment... Upon the advice of the pool shops we have tried shocking, algaecides, starver, brushing.... just about everything that everyone tells you to do. The problem is that the algae disappears for a short while and then quickly returns again.
My chlorine levels have usually been somewhere between 3 and 6, and PH is always been around 7.5. The automated dosing system is working fine.
I finally gave up on pool shop advice and decided to start educating myself. After reading through the information on the BBB method on this site, I think I've finally worked out why I've had such a major problem, and I thought I'd bounce my theory off you guys for comment...
Around 9 months ago I changed pool shops as the original shop was now under new management. It was obvious that the new owner did not understand pool chemistry so I moved on. The new pool shop used a different testing method (strips) and insisted that there was no CYA in my pool, despite the fact that 1 week earlier CYA was around 30. So we added a significant quantity of CYA on two separate occasions (the test strips gave weird results on CYA). I recently moved on to a third pool shop (who also used the test strips, and their readings were 0 CYA!!! I took the same sample back to the first pool shop and CYA was now 100!)
To cut a long story short I've recently had my water tested using a proper reliable test kit and CYA was in fact around 100.
I'm now convinced that my CYA has been sitting around 100 for a couple of months now, and despite being winter, shocking & brushing, algae is still laughing at me!
According to the Chlorine/CYA charts, because of the high CYA levels, my chlorine should be much higher than the 3-6 range that I've had it at... I can only conclude that the real problem has not been chlorine, phosphate or PH levels, but the high CYA levels!
Am I correct?
(I can't believe that the pool shops did not pick this up! I remember one pool shop telling me that it was a good thing to have high CYA - "the more the better"..)
Do I start dumping water to resolve my problem? My theory is that I need to get CYA down to around 20-30 and then I can shock the pool and rid algae for ever!