It is a mineral deficiency. Magnesium deficiency causes lowe r leaves to have inter veinal chlorosis(yellowing). Iron causes the younger leaves. In this picture, almost all leaves have.

What is the pH of the nutrient solution?

The GH flora should be providing enough Mg for growth at this stage. Iron def typically shows up at the base of the leaf and this looks more like an overall lack of health.
It could be PH related, my guess would be low oxygen levels causing root suffocation. Make sure you have plenty of bubbles/agitation and a cool nute temp in the bucket to maximise the dissolved oxygen content.

Ph is around 6.3. I am wondering if the nute temp is the issue. The tent is sitting in an uninsulated garage and has been getting pretty hot lately. I have seen temps inside the tent reach into the mid 90s. I have a fan exhausting the tent but since the garage is so hot it doesnt really do much. Maybe I should look into bringing the tent inside where temps are more consistent.

It has been a few days so I thought I would post an update. Ive got two of these cucumber plants going and both are showing the same symptoms. They were both in a nutrient solution of about 1100 ppm. Just out of curiosity I moved one into a weaker solution of around 800ppm. Both of them seem to be worsening. I am beginning to look at my air pump as a potential issue. I am running an ecoplus 18 watt single outlet commercial air pump. This is providing air to 4x 5gallon dwc buckets and 1x 10 gallon dwc tote. I read that a rule of thumb is 2 watts per gallon of nutrient solution. I have about 30 gallons of nutes so that would make my pump very underpowered. Going to hook up a couple of additional pumps.