Yellowing in the middle of the leaves

Hello ! Hello I bet since your plant are in pure water they can't get any nutrients from it ( tap water can provide Cal and Mag but that's it) , I'm surprised that they are so dark green that week compared to week one as during rooting it's generally the oposite . Your Ph range is too high , for a good rooting without soil then prefere 5.5 to 5.8 Ph level as it's the better range to root and to start a seedling but as you change your water every days you can low it daily until reach 5.5 to 5.8 ( 5.8 is the sweet spot). Another advices would be to hide light from the bottom of the glasses by wraping them in something dark to avoid any light on the roots which can lead to a longer rooting process. As soon as you will move them to their final medium and be fertilized then you should get rid of your deficiencies. I hope it will help you 😁👍.