1st grow - RQS Northern Light Auto

This week wasn't great. The plant started getting yellow edges and yellow spots, and some bugs (I suspect aphids) started to appear on some leaves. It got progressively worse with time, which I didn't really understand until I measured the pH. Having no economical means to purchase an expensive pH reader I had to use a cheaper one which only indicates the range of values in which the pH's in. I used it in runoff water and the color is as shown; I interpreted it at 5.5, which is very low. Perhaps nutrient lockout + nutrient burn is the cause for this, I don't know.