Help, my hydroponic tomato is slowly dying and losing one set of leaves every few days. The lowest branches starting to dry up one set at a time. The top leaves look healthy though. Any idea why?

No noticeable pests. Lots of water (this is a wick system) with ph of 5.5 and and 2500 PPM of the standard hydroponic plant food mix. I note that the pH is a bit low so I added more tap water which increases the pH to 6.2 and 1500 PPM.