One thing to keep in mind is the typical sand filter will filter down to 30-40 microns or so and some dead algae is actually smaller than that so it can be pushed through the filter without much effort. Some people have used DE in their sand filters to help capture the smaller algae. Have you tried that?Started with algae, killed it but best guess is pump pushed dead algae through filter to fast to filter it out.
As for the pump, a quick calculation and my guess is that the flow rate is not much beyond the design rate of the filter (44 GPM). Maybe as high as 54 GPM but certainly not enough to cause any damage.
A smaller impeller may help but as I mentioned before, there are some things a sand filter just can't remove without a little help.
If you want to see if a smaller impeller would help, one idea would be to add enough head loss to slow the flow rate down to around 44 GPM. If you have a return valve, you can try closing that until the filter pressure reads above 22 PSI. That should result in a flow rate less than 44 GPM. If the algae still makes it through the filter, it is probably too small for the filter to capture but again, DE might help with that.
Also, can you post a link to the 3/4 HP impeller? Just want to make sure it is for that model. Not all Powerflo's use the same impeller.
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