Do you prefer metric or US standard measurements? We keep shifting, lol. Seedlings want low/mid 80sF during the day and mid/upper 70sF at night. I am focused on temp because you posted a thermometer displaying mid60s F. You want an average 24hour temp of 80 not just a high of 80. The effect of low temps is slow growth. Slow growth is your current issue. I mat be misunderstanding but you see how it all seems to point to temp issues? I'm trying to wright in a clear way, without it coming across as harassment. Sorry if it did.
As for fertilizer. I am assuming you are looking for a liquid fert since you are borderline hydro with that medium. First you need a cal/mag (calcium/magnesium) mix. No liquid fert comes with those and they are the building blocks. After that is dissolved into the water you need a hydroponic fert with NPK around 10-3-6 area and includes micro nutrients. Any of the major brands will work, General hydroponics, floraGro, Fox Farm, Advanced nutrients. Others will chime in with the brands that are not coming to me right now. I use dyna-grow, foliage and bloom formulas. Ran out of General hydro mid grow so got dyna from green house down the road at a fraction of the price. Worked just as good. Been using it around 5 years now.
The absolute most important thing you need is a PH tester. Strips, drops, or meter anything will work. Test water going in and keep an eye on water PH running out. PH over 7 and under 5 the roots stop working.