So the water from the bog will stay separate from the koi pond?? Well, that's a whole different story then!! LOL!!
I just built a bog garden for carnivorous plants (pitchers, sundews and fly traps). I filled it with a 50/50 mix of sand and peat moss. Carnivorous plants like an acid, nutrient-poor mix (as do most true bog plants). I burried a soaker-hose about 5" down from the surface and punched some holes in the liner so it could not overflow into the pond it is next to. I keep it boggy, but not submerged. It looks fantastic!!
If this bog you are building is not part of the pond and does not have water flowing in it, make sure the water cannot flow into the koi pond! the water in the bog will get stagnant and anerobic and if it does go into the koi pond, you will have real problems (including dead fish). If this bog is not for filtration, I wouldn't bother filling it with any gravel or soil. Just drop the plant pots into the water, add some mosquito dunks, and you're done.