[Myka]

I wouldn't mess with the pH directly with buffers that alter the pH directly. You can use baking soda to up the pH directly, but it will fall right back down to where it wants to be. 7.8 is not an issue, no need to panic, no need to even bother to change it. A steady pH is much more important than the "proper" pH, and using baking soda or any other buffers that directly alter the pH will cause your pH to fluctuate. That's not good.

Do you have a refugium? By adding a refugium with chaeto and having the lights on when the tank lights are off will help keep the pH from falling at night. This effect will usually raise the average daily pH as a side effect.

Do you have a skimmer or calcium reactor? Often low pH is caused by high dissolved CO2 (often just from having CO2 in the house from breathing and not having open windows). A skimmer helps to off gas CO2 (an airstone will do the same thing). This is why it's also important that you don't have a greasy protein buildup on the surface of your water. Gas exchange. If you have a calcium reactor, double check that CO2 isn't escaping into the tank. I've heard of the odd person having this issue.

Another thing that will lower pH is high nutrients. If there is a large amount of organic matter breaking down in the tank and going through the nitrogen cycle your pH will drop. A good indicator is the level of phosphate particularly, but also nitrate and/or nuisance algae.

Your low alkalinity could definitely be a contributing factor. You should actually aim for about 140-145 ppm KH or 8 dKH. An alkainity of 50 ppm is under 3 dKH, which is excessively low. If you have hard corals, their health is possibly at risk if this low level is maintained.