you`ll have to provide a bit more info ;)

Looks like a potassium issue, what are you feeding them?. Judging by the upper leaves, the root temps may be a little too high and/or the rootzone O2 levels a little too low.

How do you KNOW that? I hate you. :)

Feeding them the same old Flora 1-1-1
Bubbles were lacking, hence the air stone thread.

Root temps are about 65 F.

So based on pictures I've seen...I guessed Mag and gave them a shot of Epsom.
Why do you say K?

It looks more like K than Mg.

Think like a detective :) consider. the K requirement increases rapidly prior to bloom..before the flowers appear. If you feed a low K veg nute until you see the flowers you`ll likely see a K deficiency.

If K is in high demand, its reasonable to assume that the K levels in the solution will be falling, which rules out a Mg deficiency caused by excess K. .

If you add Mg to fix a K issue, you`ll compound the problem as excess Mg reduces the availability of K in the same way as excess K reduces the availabilty of Mg :)

K deficiency

How can I quickly add K?

If your using gh flora series just replace the res with a bloom ratio not 1-1-1

Cool....thanks!

Edit: Should I do that for all my tomatoes, even if they're not having problems?

I agree. We use Masterblend which is 4-18-38 for our base. We add Blood Meal which ranges from 12/15-0-0, along with Epsom Salt (Magnesium Sulfate) and Calcium Citrate. Per 5 gallons of Reservoir (water) we add 12 grams masterblend, 12 grams Blood Meal, 6 grams Epsom Salt, and 2 grams Calcium Citrate. See our picks.

You have around 200pm of K from the masterblend which is still quite low for toms. I guess you have a lot of Ca in your source water as there`s not a lot providing it. Personally i would opt for calcium nitrate and potassium nitrate which would make adjusting N, Ca and K levels much easier. With masterblend being the sole source of K, it cant be raised without affecting the P level. At 12g/5gal P is already higher than toms need at any stage. With calcium and potassium nitrate, you could reduce the masterblend to 10.5g/5gal, increase Mg sulphate to 10g/5gal and simply alter N.Ca and K ratio`s using a combination of calcium and potassium nitrate.

Hex, it totally cleared up the issue. Thank you so much! It's amazing how quickly plants can rebound.

Nikki...thanks for that recipe! Awesome!