I happen to LOVE my Apera - it's a bit cheaper than Blue Lab - but just as accurate, just as easy, and, from others who have switch to Apera from Blue Lab, apparently longer lasting. I picked mine up from Amazon for about and it's the BEST I'VE EVER SPENT ON MY GROWS!!! Wish I had bought one when I first started growing...
As for your yellowing new growth, your instincts are spot-on... this is some micronutrient deficiency - zinc? Iron? who knows... but the key and the answer is going to be found in the PH level ... so once you get your PH pen, make sure whatever you give her (water, nutes) has a PH of 6.2 - 6.5 and you should see improvement....
Good luck!