Lone individuals may grow larger, quicker, as there is no stress from conspecifics.
I recently posted the following in another discussion .....
In fish, stress causes an increase in both cortisol and glucose levels, which in turn affects growth levels in the stressed fish. Stress increases energy consumption, which is part of the reason that sub dominant specimens do not achieve the same growth as their superior dominant siblings. Elevated cortisol levels have also been shown to not just inhibit growth, but also inhibit digestion, energy storage, and reproduction.
But there can be many different factors that can affect the growth of a fish in a closed system. Diet, water quality, water temp, 02 levels, etc. And as Rocksor mentioned even pheromones can possibly affect certain species of fish.
Having said that, plenty of people grow out fish that demonstrate good solid growth in conditions far worse, and far more cramped, than what I'm reading by the OP. Think commercial fishing ponds/vats, where hundreds/thousands of juvenile fish are raised at a time. A handful of 5" in fish in a 145 gallon tank should hardly pose any type of hormone build up in the water considering the OP's WC schedule, but there might be cortisol (stress) issues involved that are suppressing growth.
