You'll want to get an accurate reading on your soil pH. You can get a fairly decent idea by using a liquid test kit and using it to test the runoff. The pH of your mineral water is way too low. You want your soil pH to be between 6.6-6.8, and if your soil mix contains a good bit of peat (which many seedling starter soils do), then your soil pH is probably also too low. If you're wanting to use purified water, I'd recommend getting some distilled water or water purified through reverse osmosis, but you'll also need to add CalMag Plus to supplement the calcium and magnesium that purified water doesn't include. There are circumstances when tap water shouldn't be used, but 98% of the time, it's perfectly fine. Tap water usually has enough calcium and magnesium that you wouldn't need to worry about adding any CalMag. You can let it stand for 24 hours before using if you're really worried about chlorine, but I never do and don't suffer any ill effects from it.

There's also a pretty big temperature variation between lights on and lights off, which can stunt plants and cause them to stretch. Ideally, temperatures shouldn't rise much about 80F, or 26.6 Celsius. To keep the temps from dropping too drastically, you might think about vegging under a 24/0 photoperiod, which will help you to keep the temperatures consistent.

Next time you plant seedlings, you may also want to consider adding more soil to each pot. It's difficult for air to flow around the plants when they're so deep inside the pots.