Bottom set of leaves yellowing on seedlings

I'm a first time grower and have some 2 week old seedlings that the bottom set of leaves are yellowing slightly. They are approx. 3 inches tall and have 3 sets of leaves. They are in 16 oz plastic cups with holes punched in the bottom and each are under a 2200 lumen flourescents 18/6 with a fan blowing. The soil is Jiffy-mix which is 80% peat moss, 20% perlite and less than 1% lime. I water every 3 days with distilled water. I tested the N,P,K and PH of the soil with results of: PH=7.0, N= Low, P=very low, K=very low. Should I begin adding Terracycle plant food 3-2-2 and half strength Superthrive once a week? How do I lower the PH and how low should I lower it?

Thanks.

From what I understand, The bottom leaves die off (the leaves with 1,3,5 tip) later in plant growth. Your problem is probably to do with the fact you have low levels of all nutrients. Nitrogen is required for growth of stems and leaves, so try and up the nitrogen level.

you might be overwatering...you dont need to saturate seedlings so much as keep them moist....let the soil dry a little before you water them...when the soil two inches deep is dry then you should water..Over watering is a common problem with seedlings