Pls can someone help .on one plant leaves start to...

First of all, you are watering too much, I mean 1.5 liters per day? That should last the plant a week. Now let's check the rest of your info, your pH is spot on, so is't not that, your lights on temperatures are good too and so are your lights off temperatures and your humidity. Now in the picture you mention the only thing I can see are dark spots / stripes on the leaves, that usually happens due too much nitrogen. You see although your feeding was not too strong, you need to remember that you are using 18 liter of soil and that has a lot of nutrients that will carry your plant for a lot of time, for example I use 11 liter pots and that lasts me 8 weeks before showing a slight deficiency and only accepts a slight cal/mag additive to compensate for soft water (usually 0.3 cc/lt which amounts for 90 PPM (1.8 EC). In short stop the feedings of the plant is not showing any sign of needing it, I would wait at least until week 10 to start a light feeding, and get an EC / PPM meter it will help a lot in measuring how much to feed.