"I get the 4packs and what surprised me was there was only one plastic frame so every time you change the carbon and meshthingy you have to reuse the frame. At first I was a little iffy about it, but then I thought of how this saved plastic and was more eco-friendly than others. The carbon does stain the plastic, but your fish really dont care about it. Really easy to replace, just dump all the used stuff, put on a new mesh filter, then pour the carbon, wrinse and use!!"