Noob grower having problems with droopy yellow leaves

Hi everyone,
This is my first time growing, and I'm having a bit of a problem and was wondering if someone more experienced could give me some advice. Any help would be greatly appreciated!
I'm growing 2 Green O Matic feminized autoflowers in potting soil with a 42 watt CFL for each plant. I am on an 18/6 light schedule. I am watering them using tap water put through a Brita filter and then left to sit for a week or so. Whenever the soil starts looking dry, I give them about 7-8 squirts from a spray bottle, which happens about once a day. For nutes, I am using Biocanna BioVega 3.5-1.0-5.5. at about a third of the recommended strength. Every Sunday and Wednesday, I give the plants 7-8 sprays of the fertilizer mixture instead of plain water. Both of the plants appear to be in flowering stage, though one of them is about 4 days older than the other one due to seeds germinating at different speeds.
The problem is that in the past couple of days, the older plant is exhibiting droopy, yellow lower leaves. From looking at pictures online, it seems that this might be a nitrogen deficiency. Is that what it looks like, and what should I do to save my poor plant?
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That's the smallest plant I've ever seen flower