I used 3 of them on my 29 which was pretty heavily planted. I tied them all together using a check valve on each one so that you don't lose pressure in the whole line when you disconnect one to swap out. I changed one bottle every weekend in rotation. For me they seemed to produce for about 3 weeks, so that's how I ended up with that rotation.
Wow, that is exactly what I did with my 29g too. 3 bottles, one change a week during my weekend water change/maintenance.