So, an 18" sand filter => 27 GPM max flow rate for optimum filtration.

I looked up the Optiflo pumps -- they are 30 - 80 GPM pumps, which is WAY too much for that filter. I spent some time making up the image below.The original image is the Pentair data for the Optiflo pumps. I've overlaid some data for 18" SD40 and 21" SD60 Sand Dollar filters, which are similar to what you have. I got carried away with the curve, which shows the filters clean, and with a 6 psi (14' head) pressure increase as they get dirty. The pressure loss from the piping and valves is not included.





BUT . . . the curves from the pumps SHOULD cross the curves from the filter, somewhere, and they aren't even close.

What does that mean?

Most likely, instead of your filter having 27 GPM moving through it, it's probably got closer to 50 or 60 GPM! And at that flow rate, the dead algae is simply shoved right through the filter sand. It's the classic OVERSIZED pump, UNDERSIZED filter problem so common with AG pools.

And that problem typically shows up exactly like yours has.

The simplest solution is to install a ball valve between the pump and the filter, that allows you to throttle the flow back. Contrary to what most people think, this will actually REDUCE the amount of electricity your pump consumes.

The best solution is to replace your existing pump with this one:
Hayward SP15922S Power-Flo Matrix Above-Ground Pump 1HP 2-Speed @ Amazon
and then run on low speed except when vacuuming or backwashing.

OK. You can't get a pump immediately, so here are some immediate steps.

1. Do the DE test, to make sure that the sand hasn't been washed out of your pump during backwash. (This happens routinely in the BIG pump - little filter situation.)

2. Repair the filter (add sand?) as needed.

3. Install a ball valve between the pump and filter, and throttle the flow back so the filter shows 12 psi or less when clean.