It is possible that the manifold is collapsing enough to restrict water flow, especially since the middle support is also uncovered at the top.

D.E. is a good way to go. I have installed several of the new Sta-Rite Modular Media D.E. filters. Pentair, who now owns Sta-Rite, has one of their own also. They use a cartridge-type system instead of conventional grids. I also have not used D.E. in over 12 years, but a cellulose fiber substitute.

It is a much safer alternative to D.E. and keeps the water just as sparkling. If you decide to go with D.E. give it a try. Don't let yourself be talked out of it by people who have never used it or who have "heard" that it doesn't work. If you don't like it, go back to D.E. No, I don't own stock in any company that manufactures or distributes the stuff, it just works well.

I put it in all my new filter installs and have not had a single complaint. If you use it with a modular media type of filter, especially, cleaning is just a matter of opening the tank and hosing the cartridge off through the drain in the tank. The product, being all natural (it is a by-product of paper manufacturing) will usually disappear in a day or two. If you wash the debris down a drain, it won't clog it as D.E. will. A backwash valve can also be installed to these filters.

Much less mess, much easier to use. Just follow the directions on the package as to how much to add. It is about 1/8 the amount of D.E. that would normally go in the filter. A 7-pound bag is the equivalent of 56 pound of D.E. Costs a little more, but well worth it.