pH in indoor reef tanks is mostly a function of ambient CO2 levels and Alkalinity. The easiest way to raise pH is to introduce more outside air into the room housing your tank by ventilation. Another good option is to run an outside air line into your skimmer (if you run one). Beyond that you are into CO2 scrubber territory.
I use an outside skimmer line in one of my tanks as it is in an area where we eat when we have folk over and pH can drop quite quickly when we are entertaining. This line helps moderate the impact of this. In my other, larger tank I don’t bother as it’s in a much bigger room that is well ventilated and won’t have this issue.
If you are in an environment where your tank room(s) don’t get much ventilation, and you can’t improve that easily (e.g. low temperature and heating) then I would start with an outside air line to your skimmer and if that doesn’t do enough consider a CO2 scrubber. pH of 7.8 isn’t really that low however so I would first determine if it was really causing a problem or not. Personally I do like keeping it closer to 8 and above, but I don’t think 7.8 is too much to worry about. Hopefully others with more experience will give you their insights.
