K deficiency? WTF is this?

So I leave for a weekend, and come back to all of the oldest leaves yellowing and having black necrotic spots on them. 5 weeks into veg. Temps: 70 - 80. RH:30%. Res temps: 65-70f. Roots are great. 3-part AN, with B vitamin supplement. Any ideas?

Posting your PH and PPM and feeding schedule would be a good idea...

<--- Baked. Sorry, PPM's were 850 and pH stays from 5.7 - 6.3

lol its all good I know how it is.....I had the same problem on my first grow a while back....Mine was from too much salt buildup on the roots/nute lockout. I flushed and although the leaves that had the necrotic spots dont recover, the problem will not spread to the upper leavers assuming your problem is on the lower leaves which I am sure it is.

So should I flush? How long and what strength of nutes should I start back on?

Yes I would flush with PHd water asap. Your in hydroton right? Go with 1/2 strength nutes and work your way up gradually, you will see progress very quickly.