Help my coco indoor grow please

Please can someone help me is my first grow I'm using coco with canna nutrients autopots. Fast and vast autos under a 900 watt led.
They are about 3 weeks old (22/1215) they were started I've started them on less than half measures of the nutrients in my flexitank as soon as I started seeing these brown marks on the bottom leaves of about 3 out of the 9 plants one is particularly bad I will post pictures. Also my ph is up and down everyday it's doing my head in keeping it between 5.5 and 6.5 I'm having to do it almost daily the leaves are dying up with brown marks on them an look like they are dying. Any help would be appreciated thanks. It's weird that the others are looking really healthy.. The tips of the leaves are literally a tiny bit brown not even 2mm tho I've read that this is not a problem?

the reason you ph keeps going up and down (which seems to be you issue) is because of the temp of your water. the stabler you water is the steadier everything else will be. when your ph is to high it will lock out nutrients that lead to other nute lock outs. which brings me to, i looks like a combo between nitrogen, and cal mag. flush with 3 times the volume of container, then with a mild nutrient water your ladies with all the necessary nutes! good luck