The prime is for the new water, there isn't access to ro/di so tap is going to be used for water changes.
Yes, because at the moment we need to worry about adding extra nutrients to the water through water changes? Wouldn't want to increase that 120+ppm nitrate by using tap for a water change. That would be awful if algae started growing.
Wouldn't the gasses be removed when vacuuming the sand? I suppose with expensive livestock, you don't really want to take that risk.
The gasses being released and killing the fish is a long standing myth. Yes gasses do build up, and yes they have some noxious gasses. However, they exit the tank far far too quickly for them to diffuse into the water in a concentration that's even remotely close enough to make livestock blink let alone kill them. Unless you're trapping the gas bubbles and running them through an atomizer you have bigger things to worry about such as ammonia being released from the substrate.
As someone that runs deep dirted substrate beds on all of my planted tanks watching these noxious fumes bubble up from my substrate every day whenever I walk past the tank much less whenever I change the water or move my plants, I have to say absolutely nothing will happen.
