7.8 is fine and is nothing to worry about. Test the pH of your tank in the AM before the lights come on and then test it again right before the light go out. Don't be suprised to see 7.8 in the AM and around 8.2 later in the day. That is common and happens with every reef tank.
Most likely the reason for your pH being on the low, but acceptable range is you have a high co2
level in the air in your house. Take a sample of your water and test it and then take it outside and let it sit for an hour or so and test the pH. If it's higher than what it was when you tested inside the house, you have a high co2
level in the inside air.
Using any buffer to raise pH is not a good idea and usually the affect is only temporary. This may help.
Low pH: Causes and Cures by Randy Holmes-Farley - Reefkeeping.com