Help needed

First, under purple light we can not judge your plant health, take pictures under natural light. Second, I cant recommend enough that if you want to feed without guessing, then get an EC / PPM meter, even a cheap one. It is much easier to recommend a level of PPM. If your yellowing is homogeneous, then it is nitrogen deficiency (lack of grow nutes), if it is more prevalent near the tips and the base still green, then it is potassium deficiency.

Yellowing at the bottom is either N deficiency or light deficiency. If you have a really week light, the bottom foliage could just be dying, not a huge problem really. Check pH and add a bit of N. If you know you just have a really underpowered light, then prune the lower foliage.