I have a filter with some Matrix in it that I use to cycle out the ammonia in my tap water prior to water changes. (Edit: my tap water has chloramine and therefore ammonia.) Between WC’s, I keep it running in a container of water (no fish), and I feed it with ammonium chloride. It works great. I have seen a people recommend cuttlefish bone as a way to keep KH up and prevent a pH crash in the event nitrification or some other process consumes your KH. Just to get a feel for things, I put some cuttlefish bone in the container, and fed the filter lots of ammonium chloride, repeatedly. I thought the cuttlefish bone would dissolve calcium carbonate into the water at a high enough rate to prevent a crash. It didn’t. The pieces that were in there dissolved at a pretty good rate (at least they’re noticeably smaller after two weeks or so) but the pH went from 7.5 to 6.4; the KH fell from 3.5 to less than 1. Maybe more cuttlefish bone would work better, or a different material, like Texas Holey Rock (love the name), or even just straight up CaCO3. Anyhow, FWIW.