while some can hit 12 inches in 3 years or less if they're given the right conditions.
Like hell they can. lol Not sure where you got that info from, but it's complete nonsense.
Clown loaches can grow at various stages, for various reasons, water quality, diet, hierarchy within the group, as well as sex of the loach. In general, dominant females tend to grow the fastest, and become the largest. Usually there will also be a runt within the group. The largest that I've seen in person were approx. 14", and as thick as my forearm. The LFS owner had them on display with his chilli red Asian aro, much like the pic above. He imported them at 10", and had them for 15 yrs before selling them for $450 CAD each.
Those giant loaches were raised in the same tap water values as all of mine over the years, pH 8.0-8.2
HTH