Seems like I've been around and around with this problem for several years. My pool water seems to have poor filtration.
First, a little back-ground.
Pool is approx. 18,000 gallons. Pump is a 1- HP whisperflo WFE-4. I recently had to install a new motor and elected to go with a 2-speed AO Smith connected to a brand new Intermatic T1000 dual timer so that I can set both the high and low speed via timers. Pump was purchased and installed by me in about 2006.
Filter, also bought and installed by me in 2006 is a Tagelus 60D sand filter. At the time this seemed like a good choice, but I'm now learning that the filter may be a little undersized for my pump. FYI the filter is rated for 60 GPM and 325# of sand.
Now, I know that a lot of people are going to tell me to check my chemistry, but I do have a Taylor 2006 test kit and I have not had trouble keeping my chemistry within the ranges. Current Chemistry today is : FC =6, CC = 0, TA = 100, CYA =40. I did not measure CH, but multiple tests usually always come back at about 200-300 and I do not have hard fill water. I normally use MOST of the principles of BBB to control water chemistry but sometimes I use powdered shock and algecides as well as pH up and pH down, but usually I try to only use Bleach, Baking Soda, and Borax along with Muratic Acid.
For this season, I did not drain and refill my pool due to excessive rain we had. I could not drain it without risking it floating. We have had enough dry weather through the years that I normally will drain the pool completely and re-fill, which takes about 2-4 days total. Also I am very constrained on time right now. I have noticed through the years that the water becomes progressively less and less clear.
I opened it to a swamp, which I was able to somewhat quickly get under control and all the chemistry in range, with the exception of the clarity.
Here is my problem. Through sites like this one and other sites I have come to the conclusion that my pump is actually pumping more water than my sand filter can handle effectively. Also when I back-wash I believe I was losing sand, which then reduced the efficiency of my filtration. I have changed the sand at least 3 times in hopes that I could achieve better water quality, and each time the water seems to improve slightly before going back cloudy.
I changed the motor to a dual speed in hopes that the slower flow would allow my filter to work better but I have not seen much improvement. I also only back-wash in low speed. I estimate that my low speed flow is in the 20-30 GPM range. I am running my pump nearly 24/7 to try and clear the water, but it remains fairly cloudy. I can just barely see the water in the shallow end ~4 ft and not at all at the deep part in the middle, which is only ~5 ft.
My water just stays cloudy, and never sparkles like it has in the past except for right after a fresh re-fill.