I'm really shocked at the amount of emphasis that has been made insinuating that not knowing you ph isn't a concern or important. It is one of the most basic (no pun intended) things to be testing for and very important to have correct & stable. Stability is very important, probably more important than specifically where the ph is as long as it's in the 8.3 range +/- a tenth. The suggestion that an aquarium's ph fluctuating between 7.8 & 8.2 is normal and acceptable is ludicrous. That situation would be very stressful to most, of not all inhabitants and even lead to death in some. There are multiple situations that could cause this wild of a fluctuation but it is absolutely not an acceptable range of fluctuation. That radical of a ph swing indicates a major system flaw and that a major imbalance is occurring, most likely affiliated with excessive Co2. PH is a measure of hydrogen ions, the scale is logarithmic meaning each point value is a multiple of 10. For example, a ph fluctuation from 8.2 to 8.3 is a 10 point change in value, but a ph fluctuation from 8.2 to 8.4 is a 100 point change in value. 7.8 to 8.2 is a 100,000 point value in hydrogen concentration. That is a major stress generator!!!!
I don't disagree with the suggestion that a ph monitoring device can be a nice piece of equipment. They are very handy to test ph at a glance but I think additional information may be helpful in helping decide weather to buy one or not. First, a lot of reference has been made about getting a ph probe, the probe is just a piece of what you need, you need a monitoring device as well. Getting a quality probe is very important Cheap probes are not very accurate. Cleaning the probe at least monthly and re calibrating the probe & monitor monthly is very important. Even a high quality probe & monitor will give you faulty readings if you don't maintain it properly. The probe wears out too, replacing the probe every 6 to 8 months is about average. They run 50 to a little under 100 bucks depending on the quality. A monitor with a old, wore out probe is not useful or reliable.
Even if you decide to buy a ph monitor and probe, you still really need a good quality ph test kit such as salifert or aquarium systems. I have a few reasons for suggesting this, such as your aquarium isn't (or at least shouldn't be) the only water your testing your ph on. Examples would be pre-testing make up water, Testing new water for water changes, testing water for freshwater dips, testing the water of animals you just took home to determine the differences between their water and yours so you can evaluate the type and length of acclimation needed. Yes you could just take the probe from one body of water to the next but besides being inconvenient, probes take a while to settle and give an accurate reading and it is my belief that they should be re calibrated if going from one body of water to the next. If for no other reason you need a ph test kit to determine the accuracy of the probe & monitor. Most budget monitors are not sophisticated enough to determine if a probe can not be calibrated and even the ones that are, it still is a good practice to run a test to compare values. If you have nothing to compare to, how do you know it's testing correctly? (you don't) I have seen people put a lot of faith in these monitoring devises but put no effort into determining their accuracy or the age / condition of the probe only to have a negative out come.
Are you testing Ammonia, nitrite and nitrates? These are important as well.