Fungus? Can't figure it out..

I just can't figure it out. My plants are grown from cuttings, they grow fine to start, then after about 3 weeks of veg, they are starting to die off. If you look from the pictures, they start to turn yellow between the leaf veins and then work their way til totally dying. From what I can tell, I'm treating them well, growing in a good coco mix, temp stays about 70 and I'm feeding them Botanicare Veg food for use with coco and adding a bit of Pirahna to the water which is from the glacier machine and PH'd to 6.0. Lights are a strip of 4 t5's. The last batch of plants I had totally went to shit, and now it looks like it's happening to my new babies now. I grew one in a hydro screen setup just so I could keep an eye on what the roots look like, and from the pic, they look pretty good to me. Also, new growth seems twisted, I've been growing for a long time and this is a recent development for me, I've never had a problem like this. Any ideas?