First time coco grow

Day 22: Yesterday I topped the little one too. The little one is almost a week behind, and it might have been too early... I kinda regret it now as I have been thinking the last coupe of days that the plants seems too grow more slowly than they started. Now I also see some light-green stripes in the middle of my biggest leaves. I think something is going wrong here, any ideas? Day 24: I have read up about on all of yours answers, and thank you so much for your comments! I think there is a magnesium deficiency and have adjusted my nutrients once again, adding more CalMag and Rhizotonic. I've found out that leaving my nute mix for an hour or so my EC levels fall, thats why I still have an acceptable EC level at the end. I did also adjust the PH level to 6.0~. I gave it to my plants last night and measured the run-off about 12 hours later. The EC level was about 1.0 so the plants seems to have gotten some nutes. The PH level is another story. The measured PH level of the run-off water and it showed: 7.0~. Is that normal? Day 26: I now think I know what's the problem. I defiantly have a Magnesium deficiency, and I think it is caused by interveinal chlorosis, caused by too high pH level in run-off water (big thank you to @Creepy_Steve). I am now lowering the pH to 5.5~ in my soloution mix, and test run-off, as recommended by @Creepy_Steve. Atm I am getting 6.5~ ph reading of run-off so I'm continuing with ph 5.5 until I read 6.0~ in run-off water. On another note, I have started LST training. I of course snapped off one stilk, and I am so mad about it. I think it was a little late with the big one, and the stem was a little too strong, thats why I unfortunately snapped off one stilk