Hm, yes you are right that does sound exactly like Karlodinium veneficum. My only reason was that under normal situations it is highly unlikely that this algae can gain access to your closed system, but as you mentioned you were close to a body of water that in fact contained the little bugger. I'm sorry for your loss, I know I would be upset too...

As far as the Koi, thank you for "clarifying" the situation. (Are all my jokes this bad?) Anyway, I really know nothing about Koi as I said, but I welcome the knowledge from an experienced and knowledgeable Koi cultivator.