Koi are hardy, and will survive for months or even a couple of years in absolutely terrible conditions. However, given that they can live for decades or even centuries, a few months or years is a drop in the bucket.
A "large canister filter" would turn a 5.5g into a whirlpool, so I somehow doubt tha tclaim.
Now, on to your question.
As I understand it, you'll be bringing this fish to your house over the holidays. Is this correct?
Can you have a pond ready, along with a convincing story of the koi's leap to its own death (they
are jumpers)? No... Didn't think so. But I had to try.
Have him do a partial water change right before you pick it up. Then, the first day, change out a tiny bit every few hours. This will acclimate it to your water supply.
Fish Forum for discussing the often very expensive Koi. These are usually kept in backyard ponds. Also see the Koi Profile
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he might change his mind on the tank size.
He would probably think twice, then!! He had 4 green spotted puffers in the same tank before the koi, but then he read that they get 6 inches long and they hate each other... 
