I always thought I had the S7M120 300 square foot filter, and that is what the "main" label on the upper tank says, but upon closer inspection there is also a sticker that says S7M400, which is 400 square feet. I assume the sticker has the correct information, and this is just the easiest way for Sta-Rite to use the same part number for the upper tank for both filters? The reason I ask is I'm shopping for some spare cartridges and I want to get the right ones. While I'm on the subject, should I get Sta-Rite cartridges, or are some of the other brands such as Unicel decent? Thanks, Don