well i believe fish release growth hormones so in a smaller tank it stunts there growth as its more concentrrated so the only way to prevent this is to do copeus amounts of undesirable water changes and still likely to stunt there growth some one add there opinion to this aswell
You have the right concept, but apparently misunderstand the levels relevant to both fish size and tank size.
All fish release these hormones but not in concentrated levels. Until the fish is visually outgrowing its tank, it will not release this hormone at levels that will ha much of an impact if any. Your regular WC schedule will be more then sufficient at their smaller size, same with any other fish. No "extra" WC will be needed for a long time.
This hormone is released in ratio to their current size, not their potential max size, as the glands grow as the fish does.
Furthermore, I am under the distinct impression that the secretion of this and other hormones, come with maturity and ate not the function of juvies. (This part I'm not 100% on, so if their happens to be an expert in the house, I'd be interested to know for sure if you have data to share).
So again, they will be fine in a
30L for a very long time. However it will not even be close to enough to house them for life.