It's a little hard to tell from your photos, but it looks like either an iron deficiency (the telltale sign of which is veins that remain green longer than the leaves), or a nitrogen deficiency (the telltale sign of which is yellow leaves starting at the bottom of the plant and moving up).
Depending on which problem you have, it may be easier or harder to fix - you have two kinds of plants in the reservoir that use nutrients at different rates, and so the peppers could be killing your tomatoes.
If your problem is iron you are going to have to try to correct it through indirect means. Obviously you should be checking your pH - if your pH is too high your plants will not be taking up iron in the right amounts, so that's a relatively easy thing to fix. Also, if your solution is cold you might not have a good root system in the tomato plants - check to make sure there are plenty of good white root fibers, and you may need to warm up your solution a bit if that is the problem.
If your problem is nitrogen related, you can probably compensate by adding more potassium nitrate or calcium nitrate (although still check pH - it affects the uptake of all nutrients), as hydroponic plants can tolerate a high nitrogen ppm (do not ever use ammonium nitrate in a hydroponic system, as this will kill your plants once the ammonium builds up as there's no soil bacteria to process it).