Lawrosa;

If I understand correctly, this is your situation:

#1 - You have a sand filter with fresh genuine filter sand, and a pressure gauge.
#2 - After the sand filter, you have a cartridge filter (ie., in series with the sand filter)
This filter ALSO has a pressure gauge.
#3 - You've been adding DE to the skimmer, so it ends up on the sand filter.
#4 - The pressure gauge on the on the CARTRIDGE filter has been dropping after a 12 hour run.
#5 - You didn't mention the pressure before / after on the sand filter, EXCEPT that you add enough DE to cause it to increase by 1#

Here's what I think is happening.

Once you add the DE, the pressure on the sand filter goes up at once. Because your sand filter IS working, the DE stops there, and does not reach the cartridge filter. However, you notice the pressure on cartridge filter DECREASES overnight. What you probably did not notice was that the pressure on the SAND filter increased over that same interval. This reduced flow to the cartridge filter, which would -- absent other effects -- reduce pressure.

What this indicates is that the sand filter PLUS the DE is filtering everything there is to filter. IF you add the DE to the sand filter, there's no need for the cartridge filter. But a DE coated sand filter is a *TINY*, very low dirt capacity DE filter. You might be better off to EITHER remove the cartridge OR not add DE. Without DE, the sand filter will get the big stuff, and the cartridge will get smaller stuff.

All that said, I'm not sure why your pool is not cleaning up. But, I've seen this problem before, locally, didn't understand it then, either and have noted that it has been associated with use of goop, like clarifiers. But let me be 'perfectly' clear: I do NOT know that the clarifier is the problem; I don't know WHAT the problem is.

What I can tell you is that I have ALWAYS been able to clean up pools quickly, once the algae was dead, with either sand filters or DE filters as long as they were in good condition. However, I don't use (or haven't for the last 20 years) use goop. Quickly, in this situation, equals 2 - 5 days.

One of the problems with polymers, algaecides and so on is that you end up with an electrical mishmash. It's possible to combine anionic and cationic and non-ionic polymers in such a way that you can't filter anything. But, because the chem companies don't want to talk about risks OR about the details of their "proprietary" products, this can happen without warning. Also, there are several ways to precipitate colloidal (= very fine, non-filterable) particles. The HEDP metal & stain products are particularly risky this way.

Have you done this, in your pool? Dunno.
If you just keep filtering, how long will it take to clear? Dunno.
Is there anything you can do to speed it up? Short of setting up an inground pump with an inground DE filter on a separate loop, probably not.
Should you drain? Dunno.

I can tell you what I'd probably do, if I was taking care of the pool, but I'm uncertain enough that I'm ONLY offering this as an example, not a recommendation. Anyhow:

#1 - Backwash the sand filter and do NOT add DE.
#2 - Clean the cartridge.
#3 - Put both in service.
#4 - Run the pump on high for 48 hours, while maintaining chlorine levels.
#5 - Add NOTHING else, except chlorine.
#6 - If there's no visible improvement after 2 days, drain (as much as you can safely) and refill.

Sorry I couldn't help more


Ben