Seems to be getting worse! Feed more or less? HALP

I only feed every 3 waterings, its seed starting MG soil so it barely has its own. The two plants are in identical pots soil and are fed and watered at the same times. One, outside a few slightly brown tips is lush and green, the other is get necrotic patches that curl some leaves and a rusting pattern progressing from the tips. What do yall think? Not sure why one is doing fine and the other is fading away. It seems hard for me to see how it would be nute burn seeing I only feed ever 3 waterings and never more than half strength. Here's some pictures of the symptoms and one of the partner plant thats doing fine. This about 15 days into flower. The healthy one is really reaching up like crazy, the UV index in my area FL is in the instant skin cancer range today. HELPS :/

How often are you watering and how hot is it in the sun?

Ive got the watering down, only do it when its almost completely dry, usually takes three days sometime 4. Their in the sun cause its sunny today, just to save some kwhrs, and to take pics. Normally theyre indoors under cfls, 8 26w 2700ks atm.