Calcium Magnesium Supplement boosters

Do I need a Calcium Magnesium Supplement?

Your plants will tell you if they are in need of it.
If you are using a good nutrient and good water and fresh quality coco, there should not be any need for Cal-Mag.  
Some strains of plant will need it despite everyone's best nutrient designs...

A lot of growers using Coco Coir growing media will need a supplement.
particularly if

1. you use a cheap coco which isn't buffered sufficiently or designed for Hydroponics
2. you are reusing your coco so the buffering is no longer in the media

See https://www.canna.com.au/info-courier_calcium and https://www.canna.com.au/info-courier_magnesium for an in-depth look at these deficiencies.

What Cal-Mag supplement should I use?

We recommend Grow research Calcium Magnesium supplement as it is easiest to use at 1ml per litre.
However most brands will be sufficient for your needs.