First Grow. Black Tuna x2

My best advice for you is that you go out and get yourself a PH reader. You don't really need a EC/PPM meter if you are using R.O water because R.O water's PPM is like next to 0 . When you start your feeding which you should soon after that tiny second set develops I would start them at 1/4 strength this way you can judge your plants reaction to the nutrient solution and make adjustments as needed , I see your using green planet's line up . If this is the case you can be using root builder and Ocean magic from them right now . Both of these products help with root building as well as nutrient uptake leading into faster and more robust growth . I might also recommend that when its time you start using nutrients that you add CalMg to that solution this way you can prevent the yellowing of the veins in the plant as well as the purpling or all the little stems . The idea is only to really start using nutrients when you see your plant suffering from the start of deficiency's , you're medium should have a base of 1-1-1 which is plenty for the first 2 to 3 weeks even . The main thing you want to remember when they are young like this is not to over water them because when they are young it's really easy to do so . At this stage of their life's less is more . As they get older you can start to up the dosages of your nutrient solution to more appropriate strength . You may also consider wanting to avoid watering them with nutrients every time . This will do 2 things for you a) ensure your plants use up most of the nutrients in the medium before you add new ones in and b) will prevent the soil from getting "hot" and burning your roots leading to what we see as nutrient burn. In between nutrient feedings you wanna feed with PH corrected water so that your plant don't suffer from any stresses or un wanted stunts in growth as well as get any sort of nutrient lock outs which can lead to problems such as nutrient burns . Over all your plants are looking really good and off to a nice start they are a nice colour of green and they aren't stretched to a mile long . I hope this information helps you to understand better when to start using nutrient solutions and how to start them off to dial them in according to your plants needs ! -Happy Growing!