Week old Auto yellowing...

My plants pushed out of the dirt on the 14th. My auto flower now has started growing its 5-leaves, but has started yellowing heavily halfway down its 3-leaves....

All my plants are planted in your standard nursery flay, 1/2 x 1/2 x 2.

It is larger than the others, so I was wondering if this may be a rootbounding issue, but certainly it hasnt grown that quickly in its first week.

I also use soilless mix, absolutely no fertilizer in my soil. The plant couldnt have used up its own supply in only a week could it?

So what do you guys think the issue is? I will repot today just to make sure that isnt the issue, but also I have no fertilizer available atm because I thought I had more time.

However I do have a 20 gallon fish aquarium that requires cleaning today, is very green and barely seethrough. Freshwater. If it is a minor N deffieciency could I water with this for a simple fix?

All help is greatly appreciated.

This post has been here all day and I eed to do my adjustments by 1 am... any help will be rep'd.

sounds like a small N def, dont be to concerned dude its early days, as long as the new growth is healthy your sweet. think of getting some nutes.

Have you been using regular tap water? high chlorine levels may be the culprit. Try using bottled water the next few days.