Normally, on most mechanical devices, when a gasket is leaking, especially a new gasket, it comes down to a series of possibilities. These are very general.
1) The gasket is installed so it's pinched and not exactly in place.
2) The gasket is installed upside down.
3) The gasket is supposed to use gasket cement and doesn't, or isn't supposed to use cement and does.
4) The gasket appears to be the right gasket but is, in fact, the wrong one.
5) The gasket mounting surface on the device is damaged.
6) One or both of the gasket mounting surfaces is warped (happens in engines all the time).
7) The mounting screws are under-torqued, over-torqued, or torqued in the wrong order.
8) There's a defective new gasket, or even a run of bad new gaskets.

The only other things that hit me is perhaps there's a back-pressure somewhere in the system,creating excessive pressure that causes the gaskets to fail, or, Ben's idea that the excessive chlorine in the pump breaks down the gasket.

I don't know if this helps.

Carl