Whats wrong??

Whether or not to add calmag is more or less whether your tap water is hard or soft; soft water generally has a lot less ca and mg, while harder water can have fairly high levels in the water itself, so there is no need to supplement at all in some cases. Symptoms look like mg intervienal necrosis of primarily old growth, calcium excess is antagonising Mg, 0.9 is a little on the high side for a 20-day-old little one, judging by its size, it's barely reached the technical definition of "veg" period, its still growing, its not "too high" but its not going to help with osmosis, bulk flow will be reduced, first nutrient affected will be calcium. Temps are good, RH% is good for daytime, but make sure the rh is not drifting upwards of 70rh and just sitting there all night after night, transpiration is the daytime pulling of water through the roots up and out the leaves, until the roots are big enough to fill the pot and effectively "drain" the water, you need to ensure there is at least some form of evaporation taking place until the roots fill the pot. Glcuk

based on description of your watering, i'd wager this is related to poor watering habits. this is soilless with 'hydro' nutes. Fix your fertigation habits before adjusting formula unless it takes a serious nosedive. 1) Fully saturate with 10% runoff. You do not choose the volume to give. You give he volume required to accomplish the task with 10% runoff waste water. Always fertilize with a well-balanced formula relative to stage of life. 2) wait for appropriate dryback and repeat. It shouldn't be more complicated than that. If you stick with a bottom feed, you'll have to learn how often to do a top-down watering with runoff through trial and error. Is that a bottom feed setup? If so it basically eliminates one of the primary benefits of soilless growing. Bottom-feed is better for a soil. With soilless, you'd have to learn through trial and error how often to run some water top-down with runoff to avoid nutrients concentration building up in the medium. The 10% runoff, if religious about it, eliminates this as a possiblitiy, which makes diagnosing easier... symptoms are only a matter of adjusting the formula as opposed to some unseen trend occuring in the medium. I'd fix the fertigation before any major adjustments to make sure it's not a matter of the poor watering habits causing a lurking problem that has become visible.

DAT IS NOT A CALCIUM DEFICIENCY LIKE DA FRUITGROWER SAYS!!! CALCIUM IS AN IMMOBILE NUTRIENT!!! CALCIUM DEFICIENCY WILL SHOW UP IN DA NEW GROWTH!!!! DIS IS A SIGN OF A MOBILE DEFICIENCY FROM LACK OF NUTRIENTS EXAMPLE MAGNESIUM! NUTRIENTS CAN BE LACKING BY PH ISSUES AND ALSO LOW DOSES!! IN MY OPINION YOU NEED TO CHECK DA PH AND ALSO UP DA FEED!!!

I agree I think something is causing calcium lockout. Most likely some sort of watering issue. but could be excess of another nutrient taking up all the available space for nutrients. check your Ph and make sure it is optimal for your growing medium. I would get some sort of calcium supplement and see if that help. Looks like the beginning stages, so don't change more than one variable at a time to pin point what it could be. A perfect plant is rare and your always going to see something not looking too good. Sometimes those lower leaves just like to sacrifice themselves. I see your plant is clawing a little. is usually a sign of watering too often ,and maybe too high humidity compounding the problem. that plant cant transpire the moisture adequately

Once you correct calcium deficiency, the affected leaves don't heal themselves, you'll just notice it won't affect any others or any new growth

Looks to me like calcium issues most probably lock out, pH out, I wouldn't say lack off nutrients the leaves are lush green so all is good there, showing excess nitrogen slightly if anything, telling me there's to much salt in the medium, try just water and calmag for next few feeds and go from there I wouldn't add more nutrients it's defo not lack off nutes.

DAT IS A SIGN OF LACK OF NUTRIENTS!!! YOU NEED TO UP YOUR FEED!!!! MAKE SURE DAT YOU PH IS PROPER FOR BEST NUTIENT ABSORPTION DAT WOULD BE VERY VERY GOOD!!!