I am not entirely sold on calcium carbonate. I have used it a few times but I really don't see the difference in clarity. Maybe it's just me. I might have to try it again possibly with added bacteria.
It might have nothing to do with you but where and how the clams are harvested. Most will eventually die because of some unkwown bacterial/viral infection.
Just do a gravity fed float valve. Haven't had any flooding at all. Even if float valve gets stuck, the sump runs dry and you can see air bubbles on the return because of low volume.