Confused need help

Hello all,
Im on my first grow right now and its a dwc with 2 Aks.
I have been seeing alot of pics on here of plants that are 4 weeks old and are atleast a foot tall. Mine are now three weeks and are only about 4 inches.
Also the leaves look a tiny bit yellow but are very perky and growing good.

Im using a 400hps and its in my closet.

My water has been getting kinda warm could thatt be the reason for stunted growth or am i just over analyzing everything?

your vegging under an hps? that spectrum of light is good for flowering but for vegging you need something more in the blue spectrum of things, try to get a mh conversion bulb for your hps set up. you can veg under the hps but it won't be the kind of growth you see in the pictures pluse monitor your ph and ppms in the res. if the water is getting too hot you might need to put something over the top and around the tub to keep the light off of it. try using silver duct tape to try and reflect the heat/light off the res.

Are all your leafs yellowing or just the very bottom ones? If they are all, then you probably going to need to start feeding them.

Warm water has less oxygen in it, so therefore could be a factor for stunted growth, but there is probably more to it then that.

Do you have nutrients?

yes I read a little yesterday and I added all my nutes yesterday.
Today I covered the tub with mylar is that OK?
Also Im adding frozen bottles of water twice a day to cool the water.

I noticed though about an hour after I added nutes I looked at my roots and they looked gumed up with all the food. I dont know if thats what it does, but as I got the water to cool down it went away.

Ninja edit. I just checked and I actuall gave you wrong info it is a 400w mh. The plants look good it just seems like they are alot smaller for their age. Its about on its fifth set of leaves and its maybe 5inches does that sound right?

the roots getting gummed up is bad, first symptoms of root rot, the water was too warm, the mylar should help.

ok thanks basically i think it was water temp cause the plants do really well when the water bottles are in.