There is a little bit of stuff that gets past. And I do mean a little bit. My water clarity has noticeably improved in the week + that it has been in service. I have a sump with sequential sock holes. I have this in the first sock hole and a regular 200 micron felt sock in the second. I've swapped it out once and it had a bit of debris in it, but really very little. Since they are still on sale, I bought a second one to replace the filter sock in the second sock location and to catch anything that my pass through the first. All in all, these are great and any bypass really isn't worth worrying about, at least for me. And ask my wife, I'm SUPER anal - wish that wasn't the case sometimes, but a a tiger can't change it's stripe. These are on sale right now at BRS, if you're considering one, jump in and buy. I'd be surprised if you regret it.