PH changes of more than 0.2 are stressful to the fish. We are always told to be careful when adding new fish to the tank to not subject them to big changes, heck, I don't know if it's true, it's just what I've read time and time again.
I think that the additional lowering of pH if the CO2 was left on should be investigated instead of taking the book's word on that. The book may not realize how much pH was lowered by adding the CO2. How much more the pH falls overnight may not be as significant as the rise you are subjecting them to now.
Wasting gas is insignificant when compared to wasting fish.
It would be easy enough to back off on the CO2, to maybe 10 or 15 ppm and do the testing to see what the actual day to night change is with the gas on. Then if it is reasonable, not rising overnight to lethal levels, then crank back up to the 20 ppm you are running now if you feel that you need that much. You need a balance.
15 gallons and 65 watts, that's a lot of light. You are a braver soul than I to run that sort of horsepower, I'm just too new to the game to do that yet.
As for my tanks, I have DIY CO2 on 2 tanks, the 20G and the 29G, currently testing the gelatine recipes, and Hagen on the 10G. The tap pH is 8.2, the tanks all run at 7.6 pH in the morning before lights on, 7.6 in the late afternoon -- I think the power filters blow off any excess CO2. Baby fish now in the 29G so somebody is happy.