This question has been asked many times before...and the correct answer is, it depends. There is no way you can give a blanket statement of a particular species of fish growing at any given linear rate over a set period of time.
It depends on the size of the grow out tank, the frequency of water change, the water quality, proper diet, how the tank is stocked, how many of the given species are growing up together and also the quality of the stock. Many of the fish in the hobby today are F2, F3 or latter. Often times the fish are interbred with sibling fish. all well and good out of the gate, but over the long haul will result in differences that set them far apart from their wild cousins.
A couple things that are fact:
In a group of small fish, one fish will grow larger and faster than the rest. The same fish grown up individually in general will not show the same growth rate.
Fish not only grow in length, but also height and girth. A fish may not be getting any "longer" but may be filling out in body mass. Thus the "___ inch per month" is a genreallity and not a rule.
Food is not the number one means of adding size to a fish. The water is the key. Big water grows big fish. Big water changes frequently will do more to add size than over feeding.
Researching what the fish eats in the wild, and trying to dupicate that as closely as possible will do more for the overall health...and growth than fiieding the standard fare.
And lastly...some fish just grow faster than others...even within the same species.
Mo
