Acid Buffer

I'd recommend peat moss in your filter way before chemicals. Safer, slower, cheaper and works better than the chemicals.

Two things to remember with most LFS… They have an ulterior motive when suggesting products ($$) and they are also bombarded by product reps and/or suppliers to actually believe in those products.

I also have a 7.8 naturural pH. I use CO2 to lower it to below 7.0ppm Pressureized CO2 is the only reliablle way of lowering pH. 7.6 pH is optimal for 30ppm (with 5.0 KH), use what online sources state!

I have used it, it's about like shooting yourself in the foot really, but then so is any attempt at lowering ph aside from Co2 injection in reality. you are already dealing with very soft water, softer than most of us like so I'd suggest making your life easy and putting your fish in the tank with tap water. the fish will like it and it will be much easier than trying to alter it.

Lowering Ph withpeat is a little better, but still not ral good. Peat releases tannins which consume KH, when Kh is depleted, it doesn't take much of a change in anything to create a Ph crash in your tank.
If you had a very high Ph level (which you don't BTW) you can mix Ro water with tap water to lower it through dillution, which is a viable way to get the job done. In your case I imagine your KH is very low already and therefore dillution could lead to depletion and create issues as well.

You tap water is somewhere between perfect and very slightly low, so lowering it really isn't something you should desire.
dave