The more info available the better the chances that the right decision will be made.

The first fish to go into a new system (brand new or renovated) will have the best growth rate because there is limited competition for the available food. As new fish are added through reproduction or otherwise the competition for food increases and growth rates slow. These are population dynamics (whole system) not individual fish growth rates. As a general concept newly stocked waters (stocked in normal numbers) have lots of room for fish growth but in 18 to 24 mths reach carrying capacity due to reproduction and growth.