Nitrogen deficient - How to correct?

I'm thinking Ive got a bit of nitrogen deficiency... Older large fan leaves are droopy, yellowing uniformly. Plants generally a green - light green colour. younger leaves for sure are darker in colour. Thin leaves but have no experience with this new strain.

I've heard Nitrate of soda... Not sure how much to mix into my reservoir.... also if i'm wrong will I hurt them?

I have been using the Floro series GH and have not been using any CalMag... But was also considering throwing 2ml/gallon in every other res top up.

PH 5.5
PPm 750
Temp 68

Aeroflo60

Watering 5min : 1min, thinking of changing to 10min : 1min to let the plants breath.

Lots of tomatoes. Thats what Im aiming for

Any help would be fantastic!!!!!!

Come on guys, someone else has experienced this..... nitrate of soda??

just add Nitrogen....

are u running a aero garden
how offten do u feed i had some diffency cuz i wasnt feeeding them eough i was doing it every 6 hours then every 3 now on for 30 mins of for 30 mins

Well its an aeroflo.....

I feed 1min on and 5min off, as the Aeroflo instructions call for 24/7, however this = high res temp due to pump.

How to add the nitrogen is my question.... should I buy nitrate soda, then how much to give? or should i increase the GH micro?

I put a fan blowing on my pump and it lowered my res temp.

Get ur water ph 6.0-6.5 and alittle bloodmeal in the corner away from stem or root, relax , they know when ur stessed, it don't help....