To run pool water thru regular water softener to decrease hardness:

I used a 1/2 hp submersible pump, connected with garden hose to water softener - use 5/8" dia hose as short as possible to get max flow thru softener. Outlet from softener was run back to pool using another short piece of garden hose just laying on the edge of pool (with a weight on top so it wouldn't move.).

You need to accurately measure the flow rate of the submersible pump thru the water softener and back out to pool - just use a 5 gallon bucket and time how long to fill up, convert to gallons/minute.

Next, you need to know hardness of pool water, and softening "capacity" of the water softener, easiest to calc if both in "grains". If I remember right, PPM = 17.1 X Grains. So if your CH water test shows 600PPM, then that equals 600/17.1 = 35.1 Grains/Gallon.

If your softener has a capacity of 35,100 grains, that means it can remove the hardness from approx 1,000 gallons of pool water. Now, you take the flow rate of your softening system and calculate how many hours it will take to run that 1,000 gallons thru. Typical flow rate may be 5 gal/min, 300 gallons/hour - so it would take a little over 3 1/3 hours to soften 1,000 gallons.

Then you would need to flush the softener - hook the inlet to tap water, outlet to run to drain. This is hte biggest hassle - switching the hoses back and forth for the flush mode. Flushing takes a few hours, then re-connect to soften mode.

I suggest you buy some of the cheap test strips that have hardness test on them, and use to check that water going to pool is nice and soft when you first start, and that after the 1,000 gallons have been ran thru, the water coming out the softener should be hard.

Gets trickier now - as you soften the pool water, your calulations will need to reflect the new average hardness of the pool. For example, if pool was 20,000 gals,and you softened 1,000 gallons to 0 hardness (it won't really hit 0), the new average hardness of the pool is 570 PPM. It's actually more intuitive to calculate it in Grains : 20,000 gal X (600 PPM/17.1 ) = ~700,000 Grains of hardness. You took out 35,000 Grains, so now there is 665,000 grains in 20,000 gallons.

So as you go along, the time cycles will get much longer - at 300PPM, it will take twice as long to remove 35,000 grains of hardness.

My pool was about 20K gallons, CH was >700, my pump ran ~ 4 gallons/minute, and it took me two weeks to soften down to 200PPM (but my tap water is 85PPM, so they didn't help)

Good luck
Mike K

ps : double check the PPM to Grain conversion - I can't remember where I found it!