I can’t speak on how others do it. But a lot are tied into a main sump that’s part of an older system.
Or they might use a sump with old bio media.
My friend that owns a fish store told me that I could keep anything alive if I did 100% water change everyday.
Also. How long are they growing the frags in the tank ? A few weeks, months is easy.
My friend also told me the 8-10 months of a new aquarium is where problems will start happening.
1.5-2 years is when things will start to smooth out.
These are just guesses of course.