I tend to use a lot of oyster shells to hold and stabilise mainly my KH but also my PH, Only having a nexus and nowhere really to add anything like this. Ive in the past always used onoin type bags in the outlet chamber, These tend to split over a short time and make a right mess when split. After much thought ive come to the idea that these shells need being more contained. This is what ive made today and ill let you know how they preform. Plus they will be easy to remove and add new shells without all that mess hopefully.