I agree with you - most fish are not spawning in our tanks (that we know of) - but what percentage of people have 2 of the same fish, properly sexed, etc. and also monitor continuously for spawning behavior. I just dont understand how 'QT' would affect spawning - It just doesn't make sense to me.Mn, I don't know as I don't have quarantined fish. But it would be nice to know. I have been asking for years but no one seems to come forward with quarantined fish that are spawning. Any fish except clowns which will spawn on a damp sponge under a rock in Bayonne New Jersey.
If no one is spawning anything (except me) this hobby is in horrendous shape and we should give it up and collect stamps as fish spawn all the time, constantly, every few weeks and they can lay millions of eggs although I never counted them.
Every bottom dweller should spawn in a home tank and every fish that makes a nest.
The only fish that won't usually spawn in a small home tank are fish that broadcast eggs as they need much water feeling comfortable to do that.
Butterflies, tangs and whale sharks may find it hard to spawn in a small tank.
But even those fish will spawn in a large enough tank.
PS - my guess (guess only) is that those companies that are breeding multiple types of fish have some kind of biosecurity/QT process - and they are raising fry. No?