Dark spots appearing on older leaves? Deficiency or Overwatering?

I'm a first time grower and I'm trying to tackle an issue with one and possibly more of my plants and I'd appreciate any help you could offer.

Here are some quick details:

• Indoor grow
• DWC with constant water recirculating from reservoir, I was tried to also have the same water dripping at the base of the plants but I have turned that off to eliminate over-watering if that is the issue
• Lighting is a fixed 12 x 32w florescent lights running 18 hours on/6 hours off
• Constant ventilation with temperatures from 66 to 79 degrees F and humidity from 30 to 48% depending on time of day
• "Old plants" are started from seed in rockwool surrounded by hydroton with no prior germination but were then moved to Sure-to-grow to eliminate hydroton mess and ph issues
• "New plants" were started in RapidRooters directly in the sure-to-grow
• Water is tap water that starts at about 7.8ph and 90-100ppm and is then ph balanced.
• Nutrients are GH Flora Bloom, Micro, and Grow in equal parts. 5 tbsp (15 tsp) of each per approx. 15 gallons of total water in system
• Current ppm is about 510 and ph is 5.8
• There does appear to be some growth inside my hoses but it doesn't seem to be affecting the plants.
• "Old plants" are auto-flowering Blue Dream and are about 2 months old
• "New plants" are feminized White Widow and are about 2-3 weeks old

My main concern is show in the pictures of "oldplant3" (I have the pictures labeled). For a while I had issues with ph and nutrients that caused the plant to stop growing almost completely and the leaves turned very yellow. After changing the water, ph balancing at 6.0, and adding nutrients (before nutrients were very minimal) as specified before, that problem seems to have been resolved resolve.

Currently this issue is this: the plant is growing (possibly slower than normal), new leaves appear fine but may be slightly lighter green that ideal. Older leaves get dark colored spots that slowly take over the entire leaf and are moving upwards. The leaves start out fine but eventually they all get it. Some of the oldest leaves are dry and brittle. This doesn't seem to be effecting all of the plants. This is most visible on "oldplant3"

"Oldplant2" is doing better but some of the leaves are a darker green than the others and I still wonder if they are getting enough nutrients.

"Oldplant1" was quite stunted by the yellowing issue but is doing a bit better now

I have checked for root rot and the roots appear a normal white color. The roots are also growing into my recirculation system.

I have looked at some of the guides and pictures floating around to help diagnose plant issues and my best guess is either "over-watering" (meaning I need more bubbles?) or calcium and magnesium deficiency.

Instead of continuing to guess, I am hoping someone here with more experience than I will be able to point me in the right direction.

A couple more pictures that wouldn't fit in the first post.

I'm still hoping someone has seen this before and just knows what it is. It's probably something basic. I have increased the nutrients a bit and I'll see if there is a difference. It's probably around 650-700 ppm now. Thinking about getting real LED lighting and a bigger air pump with bigger air stones if it will help.

Definitely calcium deficiency, and quite possibly magnesium, iron, nitrogen. You were right to increase the nute strength. Are you mixing your water thoroughly between adding the 3 different nutes?

Thank you. I appreciate the feedback.

Yes. I let the FloraMicro circulate for a while before I add in the Gro and Bloom.

From what I have seen from other posters, it looks like my plants are a lighter green than most others. Do you know if that can be caused from the nutrient deficiencies or is that just an effect of my non-ideal lighting?

In the future I don't think I'll be doing DWC again. Next time I'll probably try high pressure aeroponics and put in some real lighting.

There's an overall nutrient deficiency. The light green can be from both the deficiency and the lighting. Honestly, it makes little sense to do dwc when using weak lighting such as fluorescent. Dwc can be great if you have high intensity lighting.
Is soil not an option? It's the easiest, tastiest, and most forgiving.
A 400w hps + soil + container + nutes can be done for under $200. You can easily get 1/4 pound from this in one cycle.....paying for itself x4 on the first run.