Saltwater Aquarium Blog

how many fish in an aquarium

How many fish in an aquarium? A few things to think about beyond the inches of fish per gallon rule

How many fish

The question: How do you decide how many fish to have in an aquarium is one of the oldest, but hardest to answer questions in the saltwater aquarium hobby. For years, reef enthusiasts have sought after the holy grail of answers that could be applied to aquariums of all sizes. But in reality, the answer should be, “it depends.”

How many inches of fish per gallon?

When I started out in the hobby, the most common advice that was passed around was usually some rule-of-thumb based on a # of inches of fish per gallon of aquarium water…from what I recall…between 0.5 and 1 inch of fish per gallon. But the problem with the inches per gallon rule is…”it depends.”

Size matters

Size matters, but the problem is that the size of a saltwater fish is relative. Most of the fish you see at your local fish store are either juvenile or young adult fish that (with luck) will grow in your tank. So how are you supposed to accurately estimate how many inches of fish you have per gallon if the number is going to be totally different four, five or six months from now?

how many inches of fish per gallon

Image by Scot Akerman

It’s not about the number of inches, it’s how you use them

Another problem with the inches per gallon rule is that it doesn’t take the compatibility of the fish into consideration at all. If you have a 55-gallon aquarium, for example, you may be excited to think you could have ~27-55 inches of fish (depending on whether you embrace the 0.5 inches or 1.0 inches per gallon ‘rule’), but if the fish you want to add are jawfish and engineer gobies, your aquarium probably doesn’t have enough space for a pair of each species, (maybe 16 inches in total fish length…) even if you keep the rest of the tank empty–because both species are competing for the same space at the bottom of the aquarium.

What about incompatibility?

Another thing to think about is the relative compatibility of the saltwater aquarium fish species in your tank. If you want to keep a royal gramma and a royal dottyback in the same nano aquarium…be prepared for the fish equivalent of the ultimate fighting championship–but instead of fighting in the octagon (UFC reference), you’ll see the fight live in your living room inside of the glass rectangle. It might just be 6 inches of fish–but those fish don’t play nicely together; neither do clownfish and damselfish or countless other doomed combinations.

Ironically, the antidote for aggression (incompatibility) may actually be over-stocking the aquarium–which may seem a bit counter-intuitive. But that certainly turns the whole inches per gallon rule on its head.

Room to breathe…and swim

Another shortcoming of the inches-per-gallon rule is that each species of fish has its own requirements when it comes to space/habitat. For example, hawkfish and clownfish stay pretty close to the rockwork, for the most part, but tangs, butterflies, and angelfish will zoom around your tank like perpetual motion machines. So an inches per gallon rule falls on its proverbial face again when you think about the needs of a powder brown tang. It might grow up to be 9 inches in length (if you’re very, very good), but you shouldn’t put it in a tank less than 125 gallons, according to–and how many of us have a tank that large? (quit bragging…I see you smiling over there). The point is–for fish like tangs, that require open spaces to roam in your tank, an inches per gallon rule pretty much falls apart.

Speed kills

You could have a 70-gallon tank, but if you added 35 inches of fish at the same time, you would probably destabilize the biological filter and end up with a bunch of dead fish, instead.

What else do you want to keep besides fish?

If you want to keep sensitive corals or other invertebrates in the tank, you also have to keep that in mind, over-and-above any considerations for the number of inches of fish per gallon of aquarium water. Each fish you add to your mixed reef tank is going to add their own waste products–potentially making it more challenging for you to keep the water parameters pristine and your corals happy. You will need to purchase a high-quality salt mix and perform regular partial water changes to maintain control over the water quality.

The unfortunate answer to the question:

How many fish should you keep in your aquarium?


“That depends”

It depends on a lot of things. It would be great if a rule like an inch a gallon worked, but it doesn’t. The two most important factors are:

  1. Maintaining aquarium water conditions and
  2. Ensuring you meet the husbandry needs of your fish.

Maintaining water conditions

If you set up your aquarium system with serious filtration and nitrate removal capabilities and/or are a fanatic about doing water changes and keeping the water parameters in line with where they should be, then you can afford to stock a little heavier than an aquarist (like me) who isn’t going to have as elaborate a maintenance system.

Meeting the husbandry needs of the fish

To keep rapid swimmers like tangs, you need a large body of open water. For dwarf angels, like the flame angelfish, bicolor angelfish or coral beauty, you need a moderate amount of room. For burrowing species, like the engineer goby, you need ample floor space and a suitable substrate. For perching fish you need rocks. It is best to mix it up. have a few different types of fish to use up the available space and make all areas of the tank interesting.

If you have any doubts, it’s best to stay on the low side of things and give your fish plenty of room.


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