They are 15 days old and i only use pure waterCould be a touch of nute burn, how old is your plant? They don't need fertilizer until after 3-4 weeks from seed. If there is fertilizer in your soil, it may be too much for the seedling. Also, it is normal for the little round leaves to die and fall off after a few weeks so don't worry about those. More info is needed about your grow to make a diagnosis.
I use this soil here is the Pic of it..its not in English but i think you can understandWhat kind of soil are you using? MG if not used right can burn your seedlings, it happened to me with my entire first grow
So the soil looks good from that pic i took?IMHO, just because it's the most expensive probably doesn't mean it's the best for all phases of the plants growth. It might be too hot for seedlings...
Actually that can be problem too because when they was lil i didnt pay attention alot when i was watering them80 is not too high to grow in. My babies are growing in 87-89 degrees everyday (I live in Arizona lol). I noticed little brown spots on my leaves one day too though and it was cause when I watered I got a little water on the leaf and the water acts as a magnifying glass and cause burn spots.