CO2 doees cause ph to drop, most definitely.
CO2 most definitely causes your pH reading to drop, but what I was saying is it is a very fleeting effect. Once you stop the flow of CO2, the tank pH will instantly start to rise. This pH swing though is different in that you are not going to induce osmotic shock or cause other issues with your fish.
When I ran DIY on a tank, I just ran a bubble wall at night on a timer opposite the lights. This off gases the CO2 from the tank at night, when the plants don't need it (and in fact are releasing CO2 through respiration).
Most definitely CO2 makes your water ph drop. When you run CO2, if not saturated completely, the CO2 excess will not only drop your ph it will react with Hydrogen in the water and can potentially suffocate your fish that you have in there. There are a couple of ways to avoid this though:
1. As mentioned you can run an aerator, this will ensure that your CO2 saturates completely.
2. It is very easy to make your DIY CO2 mixture extremely potent. There is not much yeast needed to make the mixture (max 1 teaspoon per every 25 gal in your tank max). Do not make your CO2 mixture too strong.
3. Worst case scenario, if your CO2 get too too high, do an emergency water change (about 30 %) and run the aerator for the next 24 hours this will bring your CO2 levels back to normal.
Keep in mind with DIY CO2 generators, more than just CO2 is being introduced into your tank, having a natural organic reaction like this means that there are byproducts as well. When Water, CO2, and Sugar are mixed, once reacting for a prolonged period of time this causes a by- product of alcohol (fermenting). Miniscule amounts of this will go into your tank with a DIY yeast/sugar reactor, nothing to worry about though unless you plan on running the same CO2 mixture for more than 3-4 weeks.
Remember local water quality quality plays a huge role in your PH as well, no matter how much your buffer your water, eventually the water parameters are going to be close to your local water supply.
Play around with it and see what your plants and fish look once you finally set up. My advice is get in there and get your hands dirty and see what works for you
. If you have any questions, feels free to message me, im well experienced in the CO2/planted tank/Fish realm.
Good Luck