Hi!
I'm no expert, but first thing is I'd ask why did you order the Chemi-Pure? What's going on with your tank that you think you need it (and I'm not saying you don't)? What are your current parameters? What are you observations about the tank? Do you have specific concerns?
What else you put in the filter with the Chemi-Pure (if you even need it) would again depend a lot on your tank, current conditions, and goals.
I happen to LOVE these Fluval (I still call them AquaClear) HOB filters. I think these are the best HOB filters on the market, hands down, and that includes the popular SeaChem Tidal series.
If you search, you can find all kinds of mods, tips, and tricks to make those filters more efficient; they have been popular for years. You can even find 3rd party sellers like InTank that makes media baskets for at least the larger Fluval/AC HOB's. You can even trick one out as a refugium:
https://intankaquatics.com/intank-media-basket-for-fluval-aquaclear-50-hagen-aquaclear-50/
I'd suggest please follow up with some more info about your tank, and you'll get more detailed advice
Best of luck with your tank!