Just wondering what the second most common filter type is. I already know sand is number one, any guesses to number 2? Mine: I have a cartridge filter and wouldn't trade it for anything. It's not as difficult to clean as people make it out to be, and I really love the limited water loss. I am on well water with high iron content, so the less water loss the better. I will also say that the cartridge is great at clearing the iron out of the water. If I have to add a lot of water, I just fill the pool, check chlorine, and let the filter get it. You should see how brown the filter is when I clean it. I know I am risking staining, but this is just how I do it. Note to anyone who has iron problems and reads this: Use sequestrants, and don't do it like I do.