That's fine too. Canna may cause you lockouts and deficiency no matter what your tap water is. I stopped using it a decade ago because of that. However I've used water with 0.9ec with no problems. Just take your base nute to 1.7ec instead. It's unlikely you will need to run calmag with a water ec that high. Normally a harder water will carry both cal and mag, atleast in Europe anyway. If your leaves start looking really dark green, drop the calmag and see how it is without.