A friend gave me a hardly-used 60 sq ft filter that I will soon install, so I hope to get the kind of low pressures you've got, Phillbo. I suspect my 2 hp pump is the reason my pressures aren't lower. I don't see how I could let the acid clean the grids while in the tank, because when I remove the top half of the tank, the top 8-12 inches of the grids would be out of the water. Perhaps your filter is made differently?...
Hal