Installed the Unicel cartridge this evening. Will check the results in a day or two. I popped a guess strip into the water tonight, just to see what it said. Everything "looked" good, but the chlorine was low. The pool did sit in the sun all day today, but no one swam in it. I added some tonight and am currently running the pump for a few hours. The chlorine level doesn't seem to stay regulated for more than a day or two. Is this because there's not enough CYA?