Black Taro - Goldfish

I just lost 4 goldfish, and I think I'm about to lose another. I have them in an outdoor pond, which has a filter and waterfall setup. I accidentally left the hose running in my pond for about 24 hours. I also have a lot of black taro in there which I decided to cut yesterday for the first time as it was getting a bit crazy.
And when I remembered the hose I noticed 4 dead fish. I'm not sure if it was the black taro, but I think it must be as I've left the hose running before with no problems.
I have had the taro growing in there for about a year now with no problems. I can't seem to find a definite answer as to whether black taro is toxic to goldfish.
I've taken the fish out of the pond and put them in another with no black taro to see how they fare. So should I not put black taro in my pond?