Are there any common houseplants that will kill goldfish or koi?

Things like foxglove, tulips,and cala lily are a no-no (all bad for humans and pets), I'd avoid pretty much anything marked poisonous for dogs and cats as well as children as most will be bad for the fishies as well.

As for peeps saying it'll spoil the tank, what one pond has done in my area is put them in big 'ol pots, so that you can scoop out any leaves and what not that fall in.

Best of Luck!

Ooo! Look at this site, big huge list on POND POISONOUS PLANTS!

http://www.bonniesplants.com/how_to/poisonous_plan...

By small do you mean at least 600 gallons?

Technically any houseplant with leaves that are apt to fall off should be avoided. The leaves will fall into the water where they will break down.

Almost every houseplant will not work. It would make the pond dirty and the koi will die. Don't put any houseplants!

ya