I’m new to growing and I see a lot of people talking about cal/mag what is the purpose and when should I introduce if need be my lady is 9 weeks from seed today
I don’t see any cal- mag deficiency. Your plant looks healthy as hell… I’m low- key hating 
If you are using RO water you normally use cal/mag at every feeding.
I do this in addition to 1/4 tsp of Epsom in my RO water. Extra cal/mag is not a bit deal. Plant will use what it needs and it is cheap. Not enough and you have an issue. I also use Armor SI from GH which is a Silica additive. Also cant hurt to add and it helps with strong cell structure.
There’s a lot of variables. Calmag is good to have on hand, but a lot of grows don’t necessarily need it. I see a lot of people trying to use it as stand alone nutrient lately, and that’s not really right either.
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As long as your not seeing deficiencies your good to go with any nute/additive just take into account dry organics will take longer to get into the plant as your really feeding the soil not the plant. I’ve been cutting calmag in flower but I need to extend it a bit because I’ve started seeing deficiencies lately - I always would rather under feed than over feed. Much less stress and they’re hungry to take in whatever they’ve been lacking if ph is correct. Good luck great looking plants 
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Just realized I should also have mentioned I not only use RO water, but don’t use regular soil. I am using soiless Pro-Mix HP.
This is the reason I use cal/mag. With RO water there isn’t anything in the Pro-Mix that I don’t add. Sorry if I confused @Bdnwzbdboy !! 
I’m using happy frog soil wit gh micro and bloom with nirvana and that is just one plant in a 2x4x5 tent its a t4 ac infinity exhaust I think it is going well with my first grow just trying to get more info so please comment and share I’m open to all suggestions and info