1. don't use peat pots, generally, they are a pita and they dry out too easily and they don't decay fast enough when you set out the seedlings.

2. Adjust your temperature. Tomato seedlings are not at all happy in temps less than 60 degrees and do best at about 75 to 80 degrees. When the temperature is 60 or less on a regular basis, the plants cannot absorb potassium properly and develop purple stems and leaf undersides.

3. You may have a micronutrient deficiency. A photo would make it easier to tell. Just on general principles, please get a micronutrient supplement and give your seedlings an appropriate application.

DarJones

I like high output 5000K fluoro tubes, compared to the plant and aquarium. P&A is just a money making gimmick, plants need just photons, the more the better. The plant has all kinds of phytochromes that gather up the photons of all wavelengths to convert them into sugars and starch. I am not sure if that is what is ur problem, but I did notice in my own garden of roses, that the 5000K works much better than the 3000K, due to the requirement of blue light for synthesis of chlorophyll.