Nitrogen deficiency or? Please help

After searching through plants problem topics. I assume that the my plant is lack of Nitrogen. [ I attached some photos ]
Old leaves are yellowing from the inside vein. Then to the tip. New set of leaves are light green comparing to another plant.
Do the "pros" think the same? Please help me. I'm a bit panic.
My setup is hydro,indoor light on 20/4 . I flushed all the solutions and put in the pH water+solution when this first start happening. It did not stop.
Can anyone suggest the treatment for me, thanks.

still waiting bro

Nice looking Indica's ya got there... Ok I'll answer like this and ask some questions. Ph needs to run at 5.8 - 6.2 keeping it at 6.0 as much as possible in hydro. Next indica's are healthy feeders and I don't see signs of a too strong or too weak a formula so are you using a calmag product? If not run one, I like G&H's low N CALiMAGic in hydro at 3-5ml per gal depending on my ppm's from the "clear" water. What nutrient? and at what formulation strength?
New leaf growth is always a lighter green and deepens color as they grow out to size....

Your answers will hold the solution in relation to what has already been stated.....

Thanks Dr.Who for the rep. First, I live in Thailand so the solution for Hydro came in different brand, none that you were mentioned. The hydroponics shop and supply are pretty rare around here. I have only 1 brand to work with, its come with solution A(red color, the seller said it is main source of the food( Nitrogen+stuff) and solution B(green color, suppose to be additional food ( Magnesium+Zinc). I put it 4ml each per Litre. Should I buy the EC meter? Will it be more easier for me to identify the problem?
I got the pH measure paper but it is not so accurate. Thinking about buying new pH test pen too. Is it necessary?

^I'd consider a TDS meter as being more important than a PH pen, if you've got strips. PH can safely swing a bit, and if your nutes have any sort of buffering capability, they will be able to make up for the minor accuracy error.

Also, add the micros to the res first (green).

Epsom salts will work for the calmag. Start at adding 1tsp per gallon and move up in 1 tsp incr. to a total of 3 if need be..