I recommend checking out doc buds high brix kit Q&A thread or his In the lab with Doc Bud thread. Very great information in there and he sells his kit at a great price. You will never have to worry about pH issues again!
Don't waste your money on a pH pen and all that crap. You can even use general organic nutrients and not have to worry about your pH. Get yourself a fish tank bubbler and air stones. Fill up a five gal bucket of water and put your air stones in there and let it bubble until you need it. If you're using salt fertilizers like fox farms then you definitely have to pH adjust your water/solution. If you're growing organically then pH is not an issue because your soil will buffer it for you. As long as your water source is PURE or non-chlorinated and low ppM.
I have been using general organics nutrients at the moment and have not pH adjusted once and have had no deficiencies. I bubble water and then have at it.
Definitely check out doc buds kit. You will grow perfect plants with little effort. If you need a pH reader for the moment go with the litmus paper stuff at the hydro store. That will get you through your current grow, then I would definitely switch to organic or highbrix kit growing. You'll love it!