Reef tank, Saltwater aquarium, Water garden, Tropical freshwater aquarium...
All the same. Get advice from anyone you want about the right way to keep your Koi...
BUT, at all costs you should avoid forums that specialize in KOI and definitely avoid asking anybody who actually knows anything about KOI and KOI PONDS about how to keep Koi healthy and build a decent pond for that purpose.
Besides, none of us really know anything about it. We just get off telling folks whose ponds are either gone to pot already or headed there fast that they need to do better. We just make it all up as we go along. We don't really know anything at all. Just a bunch of busybodies with time on our hands so we waste it telling you that you've got a problem on your hands and then we email each other about how much fun it was to yank your chain.
Enjoy your fish while they last. I'm sure the watergarden experts will be glad to sell you replacement for the dead and dieing for as long as you are willing. They'll sell you all the bottled bugs, water treatments, algae killers, plants, and the like until you've spend enough to build a real Koi pond and still have nothing but dead fish and the same small. stinky puddle to show for it.
At least you'll have the satisfaction of knowing how much smarter you are than us...