The pH of RO water is 7.0. Period. The reason your test is showing a low pH is because RO water has no dissolved ions in it. When you measure pH, you are measuring the ratio of hydrogen (H+) ions to hydroxide (OH-) ions. If the solution is acidic, then there will be a lot of hydrogen ions. If the solution is basic, then there will be a lot of hydroxide ions...
When water is pure and doesn't have anything dissolved in it, the pH can have some erratic readings. Many times it will show a low pH. Think of it like a bunch of magnets floating in space. They will float in random directions until you get them close to other magnets. Once they get close to each other, the poles will line up to each other...
When you dissolve something in water, it causes the magnets to stop spinning in random directions and causes them to line up together. Depending on the charge of the substance, it will either attract more hydrogen (H+) ions or more hydroxide (OH-) ions. Depending on which one it attracts more of is what determines the pH. When water is pure and has no ions dissolved in it to bond to it, many time the hydrogen ion will migrate out of solution and become dominate. This causes the water to appear acidic when it really is not. You won't really know the actual pH of the water until you dissolve something in it.
Dissolved substances act as a buffer. They cause the hydrogen and hydroxide ions to stabilize and not migrate in and out of solution. This causes a stable pH. The more dissolved substances you have in solution, the higher the alkalinity. In saltwater, it's the alkalinity which causes a stable pH. The problem is that a high amount of alkalinity blocks other substances from coming out of solution in your water as well. It bonds things so well that your tank has a hard time getting these substances back out. Specifically, calcium and magnesium.
Your alkalinity is pretty high. No doubt because of adding pH buffer every time you top off your tank. What is happening is that you are blocking the magnesium that is dissolved in the water from being able to come out of solution and go into your tank inhabitants. This means that even if your magnesium is high, nothing can use it... It's locked up.
You don't need to add anything to your top off water. Just add it to the tank as it is. Your pH is dropping at night because there is no photosynthesis going on at night to add oxygen to the tank. Oxygen raises the pH while CO2 lowers it. During the night, CO2 starts build up in the tank which is acidic (carbonic acid). CO2 comes from your fish...
You can raise the pH at night primarily 2 ways. Add a refugium and plant some macros in it. Turn the lights on at night and off during the day in the refugium. This will add oxygen to the water and remove CO2. Adding a skimmer will also help (if you don't have one). It exposes the tank water to air which causes the CO2 to be vented off and oxygen absorbed. Add some power heads to your tank to increase circulation and lay off the buffer for awhile.
