Not at all; use DE powder.

The ball valve has to be installed in the piping between your pump and your filter. Without seeing your piping, I can't tell you how to do it.

BUT, once your filter passes the DE test, you can continue to use DE, by adding say 2 cups after each backwash, as a filter aid. A small sand filter makes a tiny DE filter . . . but it will still filter dead algae better than than it will otherwise. But you'll probably have to backwash for 30 seconds or so, every day.

And yes, your problem is both extremely frustrating and very common. I remember many years ago, the first time I tried to clean up dead algae on an AG pool -- I couldn't figure out what was wrong. My experience had been with IG pools which tend to have bigger filters and not so big pumps, and I couldn't understand what was going wrong. I never did solve the problem, and I was very embarrassed about charging that customer for my time. But, 30 years ago, I didn't realize that the pool equipment companies were deliberately misrepresenting their products.

It's a common problem -- just this morning I posted on my Facebook page, a link about common deceptive advertising practices that affect ALL SORTS of industries.

I'm sorry there's nothing more I can do to help you. I *am* happy that we've been able to help a couple of guys, who are getting new pools right now, get BIG filters and little pumps, though. But most people don't come to the forum till AFTER they have problems.