No such thing as too much water. Only not enough oxygen. The lack of oxygen that suffocates roots has nothing to do with any form of hydrogen.

They will not consume much water or minerals when they’re small. Their consumption will increase as they grow bigger.

It’s ok to use but I wouldn’t spend money on it. Deuterium is 1 of 2 stable forms of hydrogen. It’s not very abundant in water so it’s not something I’d be concerned about removing.

Probably too high of a calcium to magnesium ratio. Too much calcium is an issue. Cut out the cal-mag and see if it improves. If Interveinal chlorosis (magnesium issue) continues, add 1 tsp of Epsom salt per gallon of water to your base nutrients (Canna).