if you want shade for the koi, you are much better off using a shade sail or a pergola over the pond. I would not put a thorny plant in with them or they may get injured. Injurys can get infected and lead to death.

I wouldn't put that plant with any fish you care about. I've never heard of anyone trying but there are many stories of sharp rocks injuring koi and a victoria has to be 100 times worse.