I have a 4-tier grow rack as well. Each tier came with own built in lights. They are 2', T-12's. I tried replacing them with 6500K bulbs but felt they were't bright enough or good enough. I am having a hard time finding 2' bulbs that will work in these fixtures. I need 'plug and play' bulbs. Feit doesn't have any. I have tried aquarium LED's. They aren't bright enough. Any recommendations?