Prety young, small plant, droopy curling leaves... help =(

So this is my first venture into growing, planted a seed about 3ish months ago, maybe less,plant is now about 7" tall, has always stood on its own, got about... 12 leaves growing, 2 big ones on bottom... 3 days ago it looked very healthy. Then a friend said making a CO2 generator out of a 2l bottle, some yeast and sugar... I put that in, and.. I'm not sure if that's what caused it (I think I did use too much yeast...) Anyways, now the leaves (all of them) that used to poke straight out are... drooping curling under... Read a bit and I hear over-watering can cause that, how much/often do I need to water a plant this age/size, or what else might be going wrong... also, nutrient wise, I'm just using FF GrowBig, anything I should be using instead?
 
2' T5 flourescent... In soil... I could be way off.. I think it's closer to 2 months... maybe 1 1/2, I'm talkin from the point the seed opened though... If that makes any diference at all... It looked SO healthy before... now its just... sad lookin =(...
 
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