1. Most fish do fine in stable pH, some prefer harder water. 7.8 is just fine.
2. pH drifts down because of waste accumulation .... NH3
(ammonia) turns into NO3
(nitrates) by giving off H+ (hydrogen ions = acid). The pH drift is a bad thing. You combat that with regular water changes and with adequate buffering in the water.
3. Your pH rise is likely carbonates from something in your tank dissolving into the water. <~7.8 is the equilibrium pH of HCO3.> It doesn't have to be shells, some rocks will do it too. <eg limestone>. This is not necessarily a bad thing. People would deliberately add crushed coral or shells into the tank to increase the buffering capacity of really soft water to prevent the pH drop. So if your pH is stable in your tank, I'd leave it alone. <When it comes to setting up a new tank, if you use the same substrate, you would be wise to let the water sit for a few days until pH stabalized, then slowly acclimatize any new fish to the new pH using the drip method ... ideally over several hours. Unless you are breeding fish, or doing discus or something like that, you can leave the pH at 7.8.>