Aqueon AT10 Internal Filter Hack
I had an unused Aqueon AT10 internal filter that I repurposed as a secondary filter on my tank. Not a bad filter, good flow, but tiny little proprietary cartridges. I was able to make some simple mods to (1) use better two layer mechanical filter media I already had available (compare the original blue to the new white in the first photo below), and (2) swap out the "biomedia grid" for Seachem Matrix.
Seems to be working great, and now has much more filtration capacity.