Multiple factors will determine how many fish you can put in a 75-gallon tank. Of course, the temperament of the fish provides one of the most influential factors because some cannot share the environment with species that are added after them because they will become territorial. There is also a risk of some larger species of fish-eating those that are smaller than them depending on their dietary habits. Furthermore, there are also certain species such as the yellow tang which cannot be housed with many other species due to the size that they grow too. If you have multiple rocks and other objects inside the tank you won't be able to add as many fish because more obstacles provide an issue regarding how easily they can swim around the tank. For larger species of fish, you must be prepared to upgrade your tank as they grow and reach adulthood particularly if you have more than one because the tank may become limiting and impractical for them. A general rule is to allow one inch of full-grown fish per gallon of water. Keeping 6 to 7 species of larger fish inside a 75-gallon tank is not advised as they will not be able to swim freely and live comfortably. They are also likely to become more territorial.