It's a logical question, because most people interested in having their own saltwater aquarium have videos and movies of underwater scenes of with schools of beautiful tropical fish, or have in public aquaria where a multitude of fish freely throughout the displays.
They also saw freshwater tanks with dozens of fish in small tanks.
Freshwater, saltwater, what's the difference?
You should be able to put lots of fish in a saltwater, like in freshwater aquariums, right?
Wrong!
Rule of Thumb
The short "Rule of Thumb" answer generally accepted in the marine aquarium hobby is: "One inch of fish per 5 gallons of system saltwater." The normal response to this answer is: "Is that all?
Why so few?"
The answer to the question is a bit longer and takes more time, which is why the shorter and easier answer is normally given.
The "allowable bio-load" for a saltwater aquarium depends on the size and efficiency of the biological filter, the efficiency of gas exchange, the water temperature, the fish species, size and mix, the type of filtration used, as well as the amount of cover or hiding places for the fish when they feel threatened.
Potentially, you can house more fish in a marine aquarium than the Rule of Thumb allows.
However, most saltwater aquariums, especially new ones with minimal filtration, lack a really efficient biological filtration system to process the fish waste.
Saltwater creatures are much more sensitive to tanks than freshwater fish.
Aquarium filtration systems that facilitate exchange to maintain dissolved oxygen levels at the peak of will support more fish than systems that maintain O2 levels below optimum .
The higher the water temperature and salinity, the less O2 the water will hold.
How Many Fish?
The types and species of fish, as well as the mix of fish and invertebrates in the aquarium, make a difference in the amount of fish that can successfully be kept.1 For example, a 6" Naso Tang will do quite well in a 55-gallon saltwater aquarium with a dozen Blue/Green Chromis while the same dozen Chromis would not fare well with a 6" Volitans Lionfish, which would eat them!
While this example exceeds the 1 inch per 5 gallons of water rule slightly, it is safer with smaller fish than with bigger fish.
A 55-gallon aquarium would best house only about 12 inches of fish, so to be safe, limit it to holding three 4-inch fish or two 6-inch long fish.
A vast majority of reef fishes require a "house" or someplace to hide either to sleep or to retreat to when they feel threatened.2 The same is true in an aquarium.
A stressed out fish will not live as long as a fish which can easily retreat to a safe place whenever they want.
Rocks and coral can be stacked to create crevices and caves for the fish to hid in.
Although effective filtration systems can be installed on a marine aquarium, most filtration systems available on the market are easy and to use and will work for small biological loads per the "rule of thumb".
With bigger and more powerful filters the number of inches of fish per gallon can exceed the Rule of Thumb.
The "One Inch per Five Gallons" Rule of Thumb may be a bit conservative for the experienced aquarist with a tank that has been running well for some time, but the novice would be wise to follow the rule.
Prevention is better than cure.