There's no easy magic cure. Just lots and lots of chlorine. The "treatments" do no good for algae.
To use DE in a sand filter do the following
1. Backwash
2. add about 1/4 to 1/3 cup DE through the skimmer.
3. Wait 30-60 minutes. If the pressure has not gone up, add some more DE.
4. Your goal is to increase PSI by 1 to 2 psi, no more.
5. If you increase it too much, just backwash to clear it and start again.
BTW, why do you think 13 psi is too high? My normal PSI is 20 and I backwash when I see it go past 25psi.
Carl
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