1st Grow DV - Need your tips

the pics dont show much detail. but they look fine to me.
looks like they are getting plenty of light. what are your temps like?

your smaller plant pheno might just be little slow genetically. nothing to worry about. 

unless you've added nutrients already, it sounds like your soil is too hot for seedlings. If you have been fertilizing, shame on you lol - give seedlings time to mature before giving them heavy doses of nutrients

it happens, no worries. I've been growing a long time and still make all kinds of mistakes. turns out my RO has needed replacement for about 2 weeks now and i've been giving the plants slimy water - shit happens.

if the soil itself is going bad, there must be something specific you've done to that plant - or not done - which is causing it issues. try to retrace your steps.

i would be giving them water only right now if i were you. check you pH, it's important as well.

yellowing can be a sign of over watering - and if you've had the soil sitting too saturated, it can cause all sorts of fungal issues.

lift the container a bit and feel how heavy the plant is fully watered. also note that the wieght is balanced. plants need watering just as the point where there is not enough water left in the soil to evenly saturate all the particles - consequently when your plant needs to be watered, the weight will feel off balance because the moisture is no longer evenly distributed.

later in flowering the off balance sensation will be even more apparent because of the weight of the buds. when the center of gravity rises, it's time to water.

hope that helps

Hello again, this plant is dieing. Today is worse :( R.I.P
Others doing well..

They all look nice and healthy, good luck with your grow.

Hello!
Update comming..

Shiva's top...