Can someone tell me this yellowing leaf is a sign of What? Am I not watering enough? I usually just wet the top layer of the soil and when is dry I wet it again
In my humble opinion, you are fine. I would say at least 4-6 weeks with no nutes, just water. Those are just the cotyledon leaves. Itās what the plant feeds on while it is getting established 
I am not seeing anything to be worried about, you should be good for another 2 weeks or so before you need to feed imho.
But am I watering enough?
I follow hellraisersās advice for watering in soil:
Pick up the pot. Get a feel for the weight when the soil is wet, and a feel for when the soil needs to be watered. Once you have that down, you can tell when you need to water by the weight of the pot. Best watering method I have found.
That is a good advice but it doesnāt work at the stage that Iām in right now. The pots are rather big and the plants small so I really only water the top layer I donāt feel any difference when is dry or wet because I donāt add a lot of waterā¦ thatās why Iām so confused about watering
Yeah, every new grower over waters. Weāre all guilty of it at some point. Just ask @R0ck
This may seem kind of drastic, but try waiting for the leaves to begin slightly wilting. Feel the pot at that point, then water. That will give you a starting point.
Ok thank you but for 3 and 5 gallons and 2 weeks old plants how much water should I bet watering more or less? Just want to have a better idea? Is there any forum talking about all the water process throughout the weeks?
At that age, Iām still in a solo cup, close to transplant. They drink a 1/2 cup of water about every 2 days.
My name is R0ck and I am an overwaterer. It has been 26 days since my last overwatering.
Oh your plants look fine. Let them grow a bit. At the 4th nodes decide if you wish to pinch the tops for bushes or have tall plants instead. Are they autos? If so dont pinch.
dont listen to the pot weighers lol, if you water that sprout enough to tell the weight difference your plant will drown. A few shot glasses of water every couple days or when the plant says feed me will do for now. Try to do your watering in a circle an inch or more away from the stem. You will promote more root development. Increase water slowly as the plant grows or when it says Im hungry.
In the future I recommend to all growers, before you plant take all the soil you plan to put in your pot and pre dampen it. Lay it all on a plastic sheet or in a plastic tub ans slowly add water and mix until it feels humid and sorta fluffy damp. Not wet. This will seriously make a huge difference in early root development.
Thereās 3 autos and 2 photos. So are the autos gonna just grow tall and skinny ?
They might? But you can bend them over and bend the branches doen to expose mire nodes and change the flow od auxins. Its just that topping generally sets a plant back a week to 10 days and we do t want that to coincide with the switch from veg to flower. That would not be ideal.
At what āageā do you top photos?
That depends on the cultuvar your are growing. Some plants grow like a Christmas tree, taller than wide. Typically sativas grow like this. Others naturally grow like a bush or shrub, indica and afganica typically do.
Here are my plants in the grow room for an example.
So after this example you must decide what method you are going to use? Scrog? Natural outdoor? Hie much space do you have? How much height? How intense is your lighting?
For scrog, most growers top at around the 4th node after 6 have shown, they tie those down and grow the ends out. Those ends are typically topped and the resulting tops can also be tied down. When growing in this way, its important to plant in the pot you will finish in as the rigging is not so easy to deal with in transplant.
Apparently my plants are 60 indica 40%sativa so Iām not sure if they will be tall or bushy I donāt know. I have a 4x4 tent thatās the space I have and about 6.5ft tall. Iām not sure what you mean by scrog (feeling so stupid atm) 
the hydro mars right now is at 50% but I did order 2 HLG 320 and they are coming Thursday 
and node is that counted like every set of new leaves? Sorry these are all new terms to me.
Scrog means screen of green. Have you seen guys and gals on this site spread their branches out and stick them through a net. Thats scrog. It offers support and spreads the pmant out to get the most effective use if light.
