I've read, I think on wetwebmedia that copepods start off nearly microscopic so they can travel through pumps no problem... only the adults are at risk. This makes sense to me as impellers don't actually make contact with the housing of the pump. I'm sure you may lose some but with that much room to grow I wouldn't worry. I'm curious about your algae scrubber, I'v been thinking of ways to fit one in a 20g sump.