What's wrong with it?

Looks like you got either pH problems, or some micro nutrient issues. If thats hydro they shouldn't have a look of over watering unless you are not providing LOTS of oxygen in the water where the roots hang. When i do RDWC i run my pH at 5.8 to 5.9 in veg, and when they begin to show flowers i raise it to 6.2

From my knowledge of plants arw how they react if id 2 guess its ph related. As your leafs are showing signs of stress by folding in & so on. + wit the mag/cal lookalike def imo its ph related. 5.5 set your sol at this giving u a wider area (buffer zone)of ph swings

Roberts below or DeepwaterGrower would be de ppl to DM and supply more details to help them help

Hard to say not knowing any information. It could be ph related. I run a ph of 5.8 in vegging. Plant behind it looks like it is too warm as well. Light is too intense. There is a few issues going on.