You are limited to 7 fish in standard mode and 8 in widescreen mode. Because of the collision avoidance routines to maintain realism you can't have more than that.
As far as changing fish, I highlight the fish name on the left that you want to change with 1 click, then double click on the fish on the right that you want there. That will insert your choice. If you choose the question mark, that is random choices and each time you start the aquarium a different fish will be used in that slot.
Welcome to the forum.
