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).