It seems like the growth slows a bit when they reaches 4" or 5", IMO. My one of my Green Severums was about 2" when I got him 2 months ago. Now he is 4" and didn't get much bigger. Then again...he doesn't eat like a pig. He only has 2 sticks a meal. Once he is full, he just swim off. My other Green Severum grows so fast within a month. Was only 1.5" when I got him. Now he is 3.5". And he eats a lot. The Gold Severum I talked about, her mouth is crooked to the right, so eating for her is a bit difficult, that's why she grows slower than most fishes.
I have found that severum does slow down at a certain point (as previously mentioned)
I have also found that the rate of growth after that inital growth spurt depends greatly on the tank size. Yes they will grow to there maximum size in a smaller tank. I have found however that it seems to take longer. They seem to adapt to their surroundings quite well.
I am also not stating my statement as some scientific fact. Just off of observations from my own breeding pairs and broods that I have had and raised over the years. As well as from converstations with others that I know who also breed and raise severums from this area.
BTW: Just for the record. I am not one of those believers in the "fish will only grow to the tank size" myth. I am well aware that this is a greatly misinformed and flat out wrong myth.
Well, I got a 2 inch Severum in June, whose fins were pratically torn to the body that has grown to about 3 inches since it has grown its fins back. My Severum was in a 20 gallon by itself for the first few weeks but is now in a 55 gallon with an Oscar and a Green Terror. I would say, if in a tank by itself, the Severum will grow about an inch every 2-3 months.