I'm no expert, but have used coco so I'll take a shot.

I found that feeding with every watering gave me the same symptoms that you are seeing.
With coco, a little dab will do ya.


Coco is a unique growing media.
Some call it "solid hydroponics" or 'ponics in a pot.
That is misleading.
It handles nutrients quite differently than real hydro.
It stores them briefly and then hands them off to the plant.
If you continue to feed faster than your plants can use, the coco will keep storing and concentrating the compounds.
Burns their little feet it does!

And the same goes for CalMag.

My tap water runs about 120 ppm and it's volcanic groundwater.
(Has very little Calcium)
And I found that 1 tsp. CalMag per Gallon, once a month, was too much.

With a 300 ppm tap water, I seriously doubt that the CalMag is even necessary in your case.

My advice?
Stop the CalMag, cut your nutrients back and alternate with plain PH'ed water.
With coco, it's best to pre-treat it, then wait until your plants indicate that they have needs again.

I wait until the new growth gets just a tad pale, then feed sparingly.
Works a treat, my plants are bigger and healthier than before, with much less feeding!
And there's no more leaf-tip burn either.

Hadda bail on coco though, my local store started carrying the cheaper, salted-up, crap.
Fixable, but too much like work.

Now using Sunshine mix #4 and am quite happy with it.

Aloha,
Weezard