I think most of us can agree that scientific info has pretty much established that a canna plants' need for Ca declines as the end of veg and the beginning of flower as the need for PK increases.
I was glad to see that Greenleaf has reduced the amount of CA and increased the PK in their 2.0 version. I ran into some Ca overload on my last grow during flower in spite of reducing the amount of MC 1.0 fed. I have been wondering about this since then and I believe I ran into the answer today.
I was talking to one my county's water chemists on the phone[just called and asked for a chemist!!]and asked him about Ca overload in plants in general[heh heh]! He pointed out something I had never thought about. He said that our county has a large amount of dissolved Ca in it on a regular basis. This is because of our location close to the Appalachian mountains. This whole area was created by glacial action which deposited huge amounts of limestone as they receded. Limestone is basically calcium carbonate and this gets dissolved in run off water and streams in the area....thus leading to high Ca in our water. This is what causes the brown stains in sinks and tubs.
His recommendation for folks who use tap water to hydrate their house plants[heh, heh again]is to be very careful is using plant foods that are high in Ca and titrate carefully. He also said this situation was prevalent from Maine to Georgia where the Appalachians run. Because of run off, this doesn't occur just in the mountainous areas......it can be in any part of the eastern states.
I thought this was very interesting and offer it as a FYI.