Plants getting worse..

I currently have 5 plants growing. Two over a month and 3 small plants.

The young plants were doing fine, then on one of them the leaves started to yellow, curl under and then die from the bottom up.

The rest of plants seemed ok..Now the young plants are doing horrible and my older two are starting to show the same symptoms.

Nitrogen problem?

I'm a total noob and this is my first grow, trying to learn as much as I can.

Here are the pics..

hi mate. Mine did the same!
I found PH was the problem and a nutrient deficency!
check your PH, get ya PH within like 6.2-6.6 and give em some veg nutes 1/4 strength.
They should bounce back no problem
Good Luck

thank you so much!

finally found a good hydro store too