I am trying to figure out how to maintain an old (late 70's) pool. It has a big old Squimquip sand filter, and right next to that is another canister about a third of the size. The canister seems to be downstream of filter heading toward the pool. It's got a pressure seal but no pressure gauge. Inside is just a basket made of very fine cloth in which a quantity of fine silt has collected. I have not opened up the sand filter yet so I'm not sure if that stuff is sand that has escaped, plaster from the pool, or something else. Can anybody tell me what this thing is for and what I can do to help it do whatever it's supposed to do?