Some very little holes appear after feeding them, ...

Oh, new indoor, that's nice. Enjoy, you'll love it, I got some RQS for my next grow too. Now, as to your question: It looks like a light nutrient burn, be careful, those leaves are looking really dark green, you are really close to Nitrogen toxicity. It seems you are using the Biobizz feeding chart right? What you might have missed is that although the chart calls for a 2 ml of Biogrow on the second week, that's for clones, somewhere in the chart says that you should start when the seedlings have 4 pair of true leaves deployed, you started rather early and with a dose that's a little too strong. I don't see you posting EC/PPM of your water and feeding so I'm assuming that you don't have one. 1 ml per liter of Biogrow is around 0.3 EC (150 PPM) plus your base water levels. If you are using tap water (usually 0,4 EC (200 PPM), you feeding of 2 ml / liter, could be close to 1 EC (500 PPM), and that's way too much for seedlings, try to stay at 0,5 to 0,6 EC (that would be around 1 ml of Biogrow) and only go up when you see the leaves going light green. I've burned seedling when giving them nutes too early and / or too often. Just give them water and let the leaves get a lighter shade of green, before feeding again. As a general rule, feed them every 7 to 10 days and adjust depending on how the foliage looks. A EC/PPM meter will go a long way in helping you have a good feeding strength for different stages.