The thing about Canna is it locks you into a 4.5% Calcium 1.2% Magnesium ratio. Nearly as much calcium as Nitrogen in the system, so there is plenty of that, but it lacks in Mag, especially during flowering.
I'm starting to dick around with the best Cal-Mag ratios and it is definitely better to use more Calcium earlier on. Later in flowering a Mag supplement is really helpful to getting things right. I have tried Cal-Mag, Huvega, Epsom Salt, and Sea Mag. Right now I am trying Bud Candy and next time I will try Flora Nectar. Of them all, I think Cal-Mag has worked the best when I reduce my Canna levels and supplement the slightest big of pk to balance the npk ratio back to where it should be after adding the 2-0-0 cal-mag.
The one reason I like cal-mag plus, more than others, is the iron is contains makes a HUGE difference even with tap water. For whatever reason Canna Coco A+B just doesn't have enough (if any) and the use of Cal-Mag plus keeps new growth green and lush and vigorous.
It you have the right tap, with a good deal of magnesium and iron already then no need to supplement. But if you run filtered water, have clean water, or just a bunch of calcium in your tap then over time you'll find that managing these cal-mag ratios, against NPK ratios, is the key when it comes to keeping the plants happy throughout life.
It is no joke that Canna Coco A+B and Cal-Mag Plus grow some seriously healthy plants in Veg. It is what happens in flowering that really proves that you need more than just those.
Honestly, I think systems like CNS 17 Coco/Soil Grow Bloom Ripe have much better Cal-Mag and NPK ratios for the given period of life and I'm trying to get more people to convert over to what I think is probably a better system for cannabis in coco than Canna is start to finish dollar for dollar.