forgot to add

Light is a 90w quad band LED (red/blue/orange/white)
Light and pump are on a 16h on 8 hour off cycle
air stone is on 24/7

If the leaves on top are turning yellow, then i'm going to go with some type of nutrient lockout or deficiency. How old is your nutrient mix?

If the leaves on the bottom are turning yellow, it could be not enough light from the upper leaves shading the lower leaves. It might also be nutrient lockout or deficiency.

Hi uafgrad,
If you mean "TDS 520" with a conversion rate of 500 I'd understand it as rather week nutrient concentration. If on the other hand you are using a /700 conversion rate instrument, your nutrient strength is clearly under 1 mS/cm which is too week.

What you most probably have there (according to nutrient strength) is a simple general lack of nutrients that shows up as so called "heavy" marginal and intervenal chlorosis. (magnesium deficiency for instance shows up as intervenal chlorosis and nitrogen deficiency in light green top leafs and yellowing older leafs)

Ps: If there is a evident lack in nutrients, there's no "lock-up" needed to get that kind of deficiency signs.

You may have a look yourself at this pic:

Here is a link that might be useful:

What lucus said, feed the poor things, I start my seedlings off at around 800ppm (/500).

So, what nutrient solution are you using? Your own or a brand? Water is 80? That can also lead to problems concerning nutrients and bacterial growth (which can use up some of the nutrient). Do you have to fight the pH or does it stay pretty consistent? Can you take a picture of the whole system and share it?

Usually TDS meters also display the actual EC. This is just an opinion, but I feel it is better to use that measurement in conversation to avoid the confusion that plagues the lack of standard in the industry (as Lucas alluded). The EC is what is measured anyway.

Sounds like a nutrient deficiency to me...What nutrient solution are you using? pH is very important when it comes to nutrient absorption so you might want to pay attention on this aspect....If the underside of the leaves are turning yellow, it will also help if you adjust lighting and do some foliar spraying....

Here is a link that might be useful: pH Manifesto Your Buried Treasure Of Growing Secrets

Try to reduce the 80F water temp if you can. There not enough information to make an accurate diagnosis, you could be using a flowering nute with virtually no N content ;)