Originally Posted by Dmaaaaax

You can use Excel with CO2.

For DiY CO2 you don't want to control it with a valve. You run the risk of accidentally closing it and having too much pressure build up in your bottle and something blowing. An alternative to this is to place your bubbler at a specific depth in your tank. If you don't get enough bubbles place it higher. If you get too much place it lower. The depth adds pressure naturally to the tube. Another alternative, is to adjust your surface tension. If you have too much CO2 add more surface movement. Keep in mind that at night you cannot turn off a DiY so you need to keep an eye out on the pH and possibly adjust your surface movement each night and morning (more at night, less during the day).

Your kH is pretty low. This is one of the reasons you get pH swings within a day or 2 of adding new water. I am going to guess that your GH was also around 2 until you added the coral and was the other reason. You can test your tap to see what your GH was. The higher GH has been helping you to keep your pH stable and this is the buffer. To raise your kH some you can add Sodium bicarbonate (Baking soda).

The buffers I was referring to was Seachem's acid buffer and akaline buffer. In combination you can get to a specific pH. They essentially raise your kH and GH similar to what you did and keeps your pH from bouncing. You get to the exact target pH by changing the ratio of acid to akaline. The akaline buffer is essentially sodium bicarbonate, potassium bicarbonate, and a few other bicarbonates.