If it is a true sump, it is just a container to hold water. I doubt there would be any "die off' unless he has stuff growing in it. You could clean it out and add it to your system and not have any cycle issues.
I assume you have an overflow and return pump to make the sumpe work with your setup?
*180 gal Display, 100 gal basement sump, 33 gal refugium, 3x250 MH, 2x160 VHO actinics, zoos, some softies, LPS & lots of acros and other SPS.
*100 gal prop tank plumbed into main system w/ 2x96 PC lights and 1x150 MH,