Strange relentless plant disease

I have been attemping growing, my plants are in early veggie, under a 2 tube t8 flourcent. I keep the area pretty clean but and I have had this problum too many times before the plants are taken over by some disease or mold/fungi that starts on the bottom leaves and works it way up consuming the plant after it stops growing. The leaves first turn yellow then brown spots form and the leave eventually crumples away and dies. I might have pics later. Sometimes the leaves don't turn yellow before brown spots form. I know a dual t8 flourecent is not enough but this is temp. i just got a 400w HPS/MH lamp imma ut them under in a 5x5 space. I have two plants left. HELP!!