Koikeeper99
Aquarium Advice FINatic
I really don't believe it was the BN that killed the clown. Especially in a tank of that size.
Can you put a pipe of some sort in the koi tank as a refuge for the the leopard?
Otherwise he will be okay with the BN. So long as everybody has their own little hidey hole to retreat to they should be fine. It's the leopard that will become aggressive to other plecos as he matures.
Just remember to acclimate him and check that the ph is not drastically different between the two tanks.
If the 10g is fully cycled then that is a perfectly acceptable option also.
Whichever tank he's in, be sure to give him his own piece of wood.
And don't sell your Koi!
I put this little cave in and he has had a nice piece of driftwood in the tank! I get wood for all my plecos.
His little corner he hangs in I put that green plastic plant in there as cover.