Canister Filter media options?

[Walla2GSP]

So I'm finally updating my single canister filter on my FOWLR to a dual chamber DIY 5 gallon filter. The first bucket will obviously run Poly and lava rock, but I'm looking for options in the second bucket. My current canister runs Biopellet XL and all-in-one in the last chamber, so if I'm going to remove that filter I need to find another options for nutrient removal. My ideas so far are bio balls, bulk crushed coral, baggy of carbon, baggy of GFO, bulk dolomite, or bulk sand. There are a thousand ways to set this up so I'm interested in your opinion even if I don't eventually go that route.

 

[Walla2GSP]

Also, anyone have cheaper pump suggestions? Just realized the DCT-6000 pump I was going to get for this set-up is not rated for external use.