HAve you checked the Canna website for a weekly feeding schedule?

On occation, I've seen those that were going into flower, get a shot of growth nutes the day of final transplant and switching, where they also get flowering nutrients. (Overdosing) Sure you didn't spray or spill something on it?

Anyway, that one leaf indicates to me a severe overdosing. Would be nice to see the whole plant, and have ya fill-out the troubleshooting form if they don't improve quickly, but that's the best I can offer without more...