Add it all using the PVC
pipe method, then lightly mix it in with the current substrate (move your fingers around in it or similar)
and your done with no real danger to the established bed.
I just last weekend removed about 10lbs of substrate because I had lots of different grain sizes. I literally sifted every ounce of substrate to get the grain size I wanted.
I still have a 2"-3" deep sand bed. My 3 Super Nasarius snails (they are huge, about 1.5" shell)
love the more uniform grain size, they can burrow much easier/faster now.
Absolutely no adverse effect on the BB at all, at least according to water tests.
One important note!!!
check the expiration date on the bag. If it expired or close, I would recommend rinsing it good first to avoid introducing dead bacteria and the byproducts into the tank.
In setting mine up I used the same Fiji pink from two different LFS
. One was close to the expiration date, the other had just been bagged a month earlier and still had 6 months+ according to the label.
there was a noticeable difference between the two in odor and how cloudy it made the water. I would avoid using "live sand" that is old without rinsing first.