What filter media(s) would you choose for this setup and why?

[68bruin]

I have this DIY filter box that sprung a leak, and after I fix the leak I want to put the best possible filter media for my particular situation.

I have a 4000 gal pond with a pressurized filter system: water comes directly to the pump from the bottom and from a skimmer, then goes to an Ultima II 10,000 gal bead filter. Water returns to the pond thru a venturi, a bottom pipe (which is "split" to give two directions of flow) and up thru this DIY filter box pictured below. The water flows up thru the box and out over a waterfall.

I used to have a pea gravel layer, then lava rock, then two 1" filter pads, and finally hyacynths on top, to complete the filtration (bio and mech) provided by this water return.

I would appreciate new suggestions and reasons why, for different types of media that you think might be even better than what I had. For example, kaldanes media covered with J-mat. I would like extra biological filtration along with water polishing.

[moikoi]

i only have Jmat in my 2nd chamber. i still have a empty 3rd chamber.

[GazKoi]

I would go for K1 with plenty of air that should do the job