As a matter of fact, your pH is not low at all- you are in coco, so you SHOULD be lower than the soil range, since coco is technically a soilless/hydroponic medium.
How many of the lower leaves are pale? And are they leaves that don't get much light? It could be Nitrogen deficiency or simply unused old leaves dying off because thye no longer serve a purpose to the plant.
It doesn't sound like Mg def.
Don't let coco ever dry out completely BTW.
One way to determine feeding strength is to up your fertilizer just a little- keep track of what you're using and creep up to the point where you see a tiny bit of scorch, then flush and from then on feed with the highest level that DIDN'T burn your plants.