It's a logical question as most people who become interested in having their own saltwater aquarium have seen videos and film of underwater ocean scenes with schools of beautiful tropical fish or have been to public aquariums where a multitude of fish swim freely throughout the displays. They have also seen freshwater aquariums with dozens of fish in small aquariums. Freshwater, saltwater, what's the difference? You should be able to put a lot of fish in a saltwater aquarium, just like in the freshwater aquariums, right? Wrong!
The short, "Rule of Thumb" answer generally accepted in the marine aquarium hobby is: "One inch of fish (measured from the nose to the base of the tail) per 5 gallons of system saltwater." The normal response to the answer is: "Is that all? Why so few?"
Potentially, you can house more fish in a marine aquarium than the Rule of Thumb allows. However most saltwater aquariums, especially new tanks with minimal filtration, lack a really efficient biological filtration system which can process the fish waste. Saltwater critters are much more sensitive to tank toxins (ammonia, nitrates, phosphates) than freshwater fish.
While super efficient filtration systems can be designed and installed on a marine tank, most of the filtration systems available on the market (hang on/in tank filters with bio wheels or small filter pads) which are easy and convenient and will work for the small biological loads that the "Rule of Thumb" allows.
The "One Inch per Five Gallons" Rule of Thumb may be a bit conservative for the experienced aquarist with a tank which has been running well for some time, but the novice would be wise to follow the rule. It's better to be safe than sorry.
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