If you want tomatoes to hold their weight better, giving them potassium sulfate and rockdust (I used basalt rockdust) will greatly strengthen the stems and plants. I'm not sure that the rockdust is strictly necessary to strengthen the stems, but calcium and silica, which are in rockdust, are supposed to help with strengthening plants, along with potassium. You should notice a significant difference within a few days, for tomatoes. I've done it many times when starting tomatoes and it strengthens the stems a lot every time. If your nitrogen is high without enough potassium to compensate, that might block the potassium and make the stems softer, too. If the plants have already fallen over, giving them these nutrients will strengthen them in their position, but it won't cause them to stand up again. What you can do is replant them deeper in an upright position, after the stems have been strengthened, and then they should be in a more sturdy, tree-like position.
Softer stems doesn't necessarily mean the plants are potassium deficient, per se, but it does mean they could use more if you want stronger stems. Some varieties of tomatoes need more potassium to prevent stem problems than others.
It looks like you're using a hydroponic system. Fortunately, potassium sulfate is water soluble and should work well with hydroponics. I'm not sure about rockdust.
The problem probably isn't so much the nutrients in the system lacking as that they were started and separated. Plants are in need of more potassium when transplanted, as it helps them to absorb more water from the roots. Plus, they might have used more nutrients from the initial soil, due to lots of plants being in it.
Extra potassium is helpful during transplant time.
However, from the look of your plants, I'm thinking more potassium would probably benefit all the tomatoes, at least. They don't look bad, but they do look like they have plenty of nitrogen, and extra potassium wouldn't hurt, I'm thinking. I would use potassium sulfate, personally, as I've had good success there, but I haven't done hydroponics.
Most hydroponic NPK fertilizers are high in potassium compared to fertilizers for soil (e.g. 4-18-38 vs 18-18-21). The third number is the K, or potassium. N is nitrogen. P is phosphorus.
Potassium is even more important when light levels are low, I've noticed. Nitrogen doesn't seem in as high of demand when light levels are low. There is such a thing as too much potassium, however.
If you use rockdust, be sure to realize that it may change your pH, which affects which nutrients are most available.