Looks like you have the 'ol "yellow shoulder" (that's really what they call it)...It's really not just the skin that is yellow. The tissue well beneath the skin is now abnormal and damaged.
Causes are an interplay of not enough potassium (K), coupled with very high temperatures, and high pH at the root zone (it's quite different from plain old sun scalding).
Vlad, I have a small aquaponics greenhouse with 3 IBC grow beds and tomatoes growing in 2 of them. At one end I have a media bed and all is well. At the other end I have a raft bed and these plants have always been 1 step ahead of the other bed. However in the last few days the raft bed plants are all wilting, almost as though they aren't getting enough water or getting too hot. The media bed still looks great. They are both in the same type of water and temperature. Any ideas what's happening?
There are a number of reasons a tomato will start wilting. Since we can rule out under watering as the cause, the next most common causes are Fusaruim wilt (fungal), Verticillium wilt, (fungal) bot are quite common, spotted wilt virus (a bit less common) and various bacterial wilts (even less common).
Take and post a few pics...there might be some visual clues as to what's going on?
I've grown toms in AP DWC troughs and don't much like it. The skins of the fruit tend to split and crack from all that water...and the water content of the fruit is very high so the toms are less flavorful (since the sugars are diluted by all that water).
This is how they look. The first one was 3-4 days ago.
Those are a single plant in the DWC bed. Attached here is the plants in the media bed.