I don’t know if I agree totally. Personally, if your existing bed has been in place for some time I wouldn’t disturb it. You’d be asking for a real mess. You could get away with adding live sand to your system, but chances are you’ll kick off a brutal cycle.
It sounds like your system has been up a while. Why not preserve the bio-activity in the current bed. If you add fine sand to the current bed eventually, as said before, much of the larger bed will works its way to the surface.
I’m just suggesting this to think about. Don’t run out and do it without getting other opinions for I may be way off. But…
You could add a layer of nylon screen (like the stuff on your windows) to the top of the current bed. Cut it so it sits ½” in from the glass so it won’t be seen from the outside. When the fine sand is on top you can mix the edges a bit so the bed doesn’t look like so divided from the outside. The current bed remains undisturbed for the most part. The trick is to install it and add enough sand on top so it won’t end up creeping out into the tank.
Depending on the depth of the current bed, I’d add at least 2” (or more) of aragonite. Wash it thoroughly before adding. Have a
mechanical filter handy to run for a few days to get rid of the resulting cloudiness.
The underlying bed is already “alive”. That will seed the upper bed. In no time at all you’ll have a lot of bioactive fine sand. After you see life, get a
sand sifter like a Sleeper Goby to maintain it for you.
Just two more cents.
Good luck!
Joe