Tap water should never be that low in pH. At that pH the water will leach copper from the copper piping, and lead from the solder used to assemble the piping. If it truly is that low, you have a very major problem, far exceeding aquarium problems.
A drop checker works to measure the CO2 in the water by having 4 dKH water in the bulb of the drop checker, with that water being distilled or RO/DI water with nothing in it but bicarbonate of soda to achieve the 4 dKH. Then, that water should have 2-3 drops of bromothymol blue pH reagent (API pH reagent, for example) added to it. It works because the ppm of CO2 in the drop checker will be the same as that in the aquarium water, if you wait long enough, and the drop checker is designed correctly. It will not work if those conditions are not true for your setup. You cannot ever use the solutions that are included with a commercial drop checker, especially one made in China. Buy 4 dKH water from an experienced planted tank hobbyist or make your own 4 dKH water from distilled water and baking soda. Use your own pH reagent assuming it is one that is yellow at around 5.5 pH and blue at around 7.5 pH.
Contrary to what so many people say here, drop checkers do work. You may find their very slow response isn't acceptable to you, but that doesn't mean they don't work. And, they don't give an accurate reading, just a good enough approximation to let you know your CO2 bubble rate is in the right ballpark. If you want an accurate reading on how much CO2 is in the water you have to spend at least hundreds of dollars. But, we have no reason to want an accurate reading.
Having read through this thread, I'm having a hard time figuring out why people are still butting heads and the problem hasn't been solved with one page of comments yet.
Hoppy and a few others have laid it out pretty simply here: The science behind how drop checkers+bromothymol blue+4dkh water and co2 levels work has been established and it works. If your drop checker doesn't change color or has the wrong color it can ONLY be one of three things.
1) bad circulation/placement of the drop checker.
2) you're not using 4dkh solution which should just be pure water adjusted to 4dkh with only carbonates/bicarbonates.
3) you're not using bromothymol blue
If it's #1, move your drop checker or increase circulation. If it's #2 or #3 figure out which reagent is the problem and replace it (ph indicators are generally bromothymol blue-esp if you use the API one. it's most likely 4dkh solution that's the culprit, most companies do not provide actual 4dkh solution or tell you to use tank water. this is incorrect
you must use legitimate 4dkh solution which you can buy from sellers in for sale/trade section of TPT or you can make it yourself with RO/DI water and baking soda)
Secondly, understand the limitations of the drop checker. It's not going to give you a precise ppm count, only general ranges of where your co2 levels are. Blue=not enough co2, Green=good amount of co2, Yellow=Too much co2/bordering deadly. For most people all they need to know is if they have enough co2 or not. Knowing your exact ppm or co2 induced pH drop won't make your plants grow any better. Probably the only reason you'd need to buy a ph monitor is as a failsafe for expensive livestock. You're paying less than 10 dollars for a glass ornament and some colored liquids, there's no use in comparing how "useful" or "accurate" it is to a ph monitor. The drop checker is for a quick visual reference to general co2 levels and there's a 1-2 hour lag time for color changes associated with co2 level change. Dialing in your tanks various parameters is a gradual process, not something you just set values to in an hour and expect to run smoothly forever.
Whether you do or do not think a drop checker is useful is your opinion which you're fully entitled to, but the science behind how drop checkers work has been established and is fact. so to each and their own...