The middle plant is HSO Blue Dream at around 30 days since seed. The others are HSO Green Crack, around 25 days since seed (the right one popped soon, that's why it's bigger than the left one). I've been feeding them around 16ml of Canna Coco A + B per 10 litres (meaning, 8ml of each). I've added a few ml of calmag a couple of times (around 5ml per 10L) but it raises the EC so much I wasn't sure how much to add.
What do you guys reckon, is it a calmag issue? I also feel like I could be overfeeding a bit too, maybe I should drop down to around 1ml per litre of A+B, and 1ml per litre of calmag. Not sure what the final EC would be on that though...
Canna Coca A + B
Around 0.8 EC
Strange one, because everything seems fine regarding feed and all that..... I would say maybe even a little more food would be good, bring the a n b up to 10ml per ten lites now, instead of 8ml and see how it goes.
There seems to be some algae on your medium? I think the plant might be getting a bit too cold, and the coco is too. Then its staying wet for too long and that's what's causing the issues. The root zone being too cold and wet.
Bring the light closer, around 18 inches from the top of the plant, and bring the fans closer too, make sure one is pointing at the top of the coco, that'll help it dry quicker.
So, light closer, fans closer, if you can add a heater to keep the grow room warmer, and also increase the a n b a little. See how they go from there 
've added a few ml of calmag a couple of times (around 5ml per 10L)
This is a decent dose of calmag too mate, 0.5ml per litre is good, but don't use it too often.
Got a fine mist spray bottle with epsom salts @ 1t/liter? If so, you would see an improvement in 24 hours if it is magnesium. Spray when lights go off. But Like Macky said, the algae is concerning. I am not a coco grower but is it fresh and screened free of the fines? It looks like it is staying too wet for too long.
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