My GH is 20mg/L and I've been told that to improve plant growth I should
provide a source of calcium and magnesium. How do I do this? ie where do I
get it from? How much do I use etc?

Other information:
PH - 7.2
Ammonia, Nitrate and Nitrite all low
Phosphate 2.5 but I'm working on this.
KH - 150mg/L
CO2 - OK
FE (Toxic) - zero
FE (Non toxic) 0.18

I weekly dose with Nutrafin Plant Gro

Thank you - Dave
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David Scott
Hampshire, England