Yes, the density will affect the growth of the fishes. We know that closed systems like your containers have a limited carry capacity (that is the key element for aquaculture and the aquarium fish business). That means there is only so much biomass (total mass of all living things) that can be sustained in a container of a given size, so the bigger the container the more fish it will support (assuming they are fed and cleaned regularly).
Out in the wild however, the answer is usually no, we don't see density affecting growth. The reason is that wild systems tend to be open with very complex inputs and outputs. When fish biomass starts to increase--that can be the number of fish or the total weight of all fish--the population of fish responds by either reducing its numbers ( leaving the area, suffering greater number of deaths from predators, lower birthrate) or they lower their growth rate, that is, they stop growing. So in the wild, we rarely ever see that density affects growth because many other things happen before these two factors affect each other.
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