Hi Mike S, and welcome to the forum!!

I use zeo in my 300# filter with a 1 hp pump, but have never seen any zeo washing out through the backwash. It will, however, blow back into the pool if not set on "rinse" mode. I wonder if maybe your pump is oversized for your filter? I'm not an equipment expert by any means, so hopefully one of them (we DO have them here, though! ) will be by shortly to comment.

By the way, thank you very much for supporting the forum with your subscription!!

Janet