I believe the cause of your troubles is the Bacquacil--it sounds like you may have the dreaded white mold threads and they are clogging it. The sand will probably go longer, but I gather you'll have to clean out the filter a couple of times a season.

Of course, if you switch to chlorine, you don't have those problems. Many cart owners with chlorine only clean the cart every month, or every two months. Sand owners don't ever have to do more than backwash--I'm four years on my sand and haven't cleaned it yet. My fellow mods will tell you my water is clean and clear, too.