Yes I certainly think both things are possible here Jeff...not enough total ppms and the plant is drawing on the older reserves to compensate.

I change things out completely once a week but draw off SOME old solution and add new at least once between then as well.

Since you are using tap water I would put them on approx 150-200ppms Pro Gro and see what they do with that. I honestly don't think you can get into too much trouble at that number. I still don't think you need any CalMag is you are using tap water but if you have Liquid Karma you could toss in 2m/gal of that as well.

Edit: let me clarify....thats 200ppms of Pro Grow IN ADDITION to whatever ppms your tap water already has in it.