Couple of thoughts...

If it's only Hydroton and you let it go dry, root damage may have occurred...

Photo 5 looks more like it's drooping from heat off the light and lack of water...

Might try watering one from the top ( using water out of your res. ) every 4 hours and see how it reacts...

Might want to back off the 600w till your figure out the problem...

Food for Thought...