I soak in a mix of vinegar and water a few hours. Usually have to scrub/pry that Seio impellar apart in the end which is risky as the shaft is cheap and cracks easily. I don't have Seios anymore but the process is the same with all pumps. Disassemble impellar, soak, scrub if needed.
I then reassemble and RUN the pump a couple hours in that water/vinegar solution.
Rinse in water or not, depending on my mood.