In my last darkroom, I put in a 10 gallon electric tank. It proved to be useless as I didn't turn it on unless I was working in the darkroom, and then it took too long to heat, and then ran out too soon. My current darkroom is plumbed for a heater, but I have yet to install one. However, I'm in Texas and you're in Canada, so things are different.

If and when I do install a heater, it will be a standard 40-50 gallon model, which I have seen used successfully in other local darkrooms. In all the years I've been on internet forums, I not heard much good about tankless in a darkroom situation. It's the flow rate.