Hi all, sorry if this has already been answered but I couldn't seem to find it. I just bought a house in Dec 2008 with a 15k gunite pool, my filter is a sta-rite system 3. I found the manual online and have looked around the internet but could not find a good answer. I wanted to know from some experienced ppl when do you know if the cartridges need to be replaced? I have no idea how old they are, they could be 6 years old for all I know, they definitely don't look that new. I only get down to 15psi after cleaning them, should I expect a lower psi? What is the general guidance to replacing those other than obvious holes and tears?