Nute help please

hey all, gotta another question. If a plant in the vegatative stage has purple/red stems and the bottom leaves continue turning yellow and dropping what nutrients is needed? im thinking more nitrogen and micronutrients but wanted to be sure first. I gave it a bit of fish extract like 5-0-1 or something like that a few weeks back and a tiny bit of 12-12-12 slow release plant food, soon after a few of the leaf tip were going yellow and brown but i ran lots of water through it and the minor problem has seemed to disappear. just cleaning up a small and newly spotted bug problem :( but ill get it sorted out. any ideas about wut nutes i should use?, and what the burning tips may have been cuased by. Im lookin for a good quality but cheap nutrient that can be picked up at my local hydroponic store, also we get lots of Green Earth growing products around here.  thx and happy growing