Whats wrong with my babies??

Hello everyone my plants are 2 weeks old.. they were growing pretty fast and one day some brown spots started showing on the leaves. I didnt give them too much importance, I thought it was water that I had sprayed and had burned down that part of the leaf.. but soon after more and more started showing and these leaves started turnning yellow, still with the brown areas. They also went very dry and skinny and if you touch those leaves now they will break. The plants are growing really slowly now.. I dont know what the problem can be.

Im noticing that more brown spots are showing on the younger leaves Ive no idea how to stop them!

I have the right temp at 25ºC and RH at 52% or so. The soil im using is a universal soil with ph 5.2-5.5 but I was told it was good enough.. and Ive been using this soil since i started growing the plants and they were growing perfectly until now..

Pictures

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THANK YOU

Looks like a Magnesium problem...? I've seen a lot of this issue lately. I wonder if it has to do with season change? Here's the link to this sites FAQ section: Text

Hope it Helps!

take photos in white light only to show color, not sodium.

get a bottle of dynagrow fertilizer at the grow shop. follow directions on the back for soil use - 1/2 teaspoon in a gallon of water. don't water too much though. transplant in a while to bigger pots. use foxfarm potting soil. go easy on the additional fert as ff soil has some already.

Hi here there's a photo of one of the plants I took today.

http://img252.imageshack.us/img252/1135/p4020048.jpg

I've added fertilizer but I dont know if I should transplant into other pots with different soil because the pots theyre in right now are the biggest I have, 7L, and are meant to be the last. Should I transplant the plants into the same pots but changing the soil?

Thank you so much