Given what was posted, I'm calculating that you have 27+ lbs of fish in your tank and that you are hitting 200+ ppm in nitrates. (That's assuming you aren't starving the fish, that you have no alternate nitrate removal, and that you are doing a 80% WC every 3 weeks.) I can see random fish deaths. IMO, the store misquoted the water quality, didn't test for nitrates, or was given water right after a large WC.
Even if you have some alternative way to reduce nitrates (in the sump?), that is far too large a bio load for that tank. And most of the fish are still growing.
Even if you have some alternative way to reduce nitrates (in the sump?), that is far too large a bio load for that tank. And most of the fish are still growing.