An alternative to a pH meter is the familiar pH test kit, where you add a few drops of dye to a sample of water and compare the color of the sample with a color standard to determine the pH. It's a perfectly workable and inexpensive solution--the only shortcomings are that it's a bit messy, it's time consuming (if you do it regularly), and it doesn't give continous results (so you can't use it to control CO2 injection).
Shown here are two kits, one from Aquarium Pharmaceuticals that sells for about $5.00 and another from LaMotte that sells for five times that, or more. There's no denying that the LaMotte kit is a very classy solution--it comes with a comparator with actual solution samples showing the pH values. You just add 8 drops of dye to the sample in the test tube, slide it into the comparator, and hold it up to the light to compare the color of the sample to the color of the standards, from which you get the pH.
People rave about the LaMotte kit, and there's no denying it's a great product--it even comes with a plastic case. I was surprised to find in side-by-side comparisons, however, that I consistently got the same pH value with the cheaper and the more expensive kits. That is, the fish store kit seems to work just as well.
Here you can see the test tube, comparator, and the brothymol blue dye test reagent. You slide the test tube with the sample into either one of the two empty slots in the comparator to get a side-by-side comparison of the color of the sample versus that of the standards.
The pH of this sample is about 7.3.