They really just need to be replaced. Things that cause scum are makeup, lotions, and other products that are introduced by the bather to the spa. Try to reduce that will really help. Many people will go as far as to rinse off prior to entering the hot tub. That seems like a bit much to me. I recommend not applying those things prior to entering.
There is also a product by Liesure Time that they call "enzyme" that works really well to avoid scum.
