Yes, spider mites aside you
really need to be able to measure and adjust your pH! Along with a good meter or the General Hydroponics water test drops (which only cost around $7 and are fairly accurate), you need a bottle each of pH UP and pH DOWN so you can get it in the correct range after you measure it. I use the water test drops and have not used a meter before, but I have heard people say good things about Hanna brand pH meters.
Just make sure you don't buy a cheap
soil test meter or kit. The only good soil pH tests/meters are expensive, and the readings from a cheap one will not be accurate enough to be useful.
You don't have to spend a lot for a good
water pH meter, but I wouldn't get one that was like only $10 or something.
Check out the Hanna brand online and read some reviews of certain meters, then find one that has good reviews in your price range. If they don't carry them at your local grow/hydro store, then you can order one from the internet.