How to Know How Many Fish You Can Place in a Fish Tank

Three Parts:Determining Your Tank’s Holding CapacityDetermining What Kinds of Fish to House TogetherMaintaining Your Tank

Perfecting the stocking level of your tank is as much of an art as a science. The aim is, generally, to know the maximum amount of fish you can keep without overcrowding. Discovering this limit requires some considerations on your part. But with a little research, you can figure out how many fish you can keep in a single fish tank at one time.

Part 1
Determining Your Tank’s Holding Capacity

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    Measure your tank. The standard measurement is liters. You should be able to figure out how big your tank is when you initially purchase it. But if you don’t know (or don’t remember) the size, you can figure it out on your own mathematically.[1]
    • To find out how many liters your tank is measure the length, width, and height of the tank in centimeters and multiply them together, then divide by 1000. This number is only approximate as the actual value will be rather less due to glass thickness, substrate, decorations, etc.
  2. 2
    Maintain proper water levels. Have the water at the right level to accurately calculate how many fish will fit. If your water levels get too low, your tank won’t be able to sustain as many fish. Typically, you should keep the water level about one inch above the lip of the tank’s water filter.[2]
    • Remember that water displacement also plays a factor here. If you have a lot of decorations in your tank, you won’t be able to house as many fish as a tank the same size with fewer decorations would.
  3. 3
    Use the gallon/liter rule. A good general rule for determining how many fish your tank can safely hold is the fish per gallon guideline. Typically, a properly maintained tank can handle one inch of fish per gallon of water.[3]
    • For metric measurements, you can reckon one gram of fish for every four liters of water.
    • This guideline works best for small community fish like tetras, rainbowfish, platies, etc.
  4. 4
    Use the surface area rule. Since the surface of the water is where gaseous exchanges happen that help sustain your fish, it is important to consider this factor when stocking your tank. A larger surface area means more oxygen exchange for the fish in the tank.[4]
    • A taller tank with the same surface area as a shorter tank isn’t able to house more fish even though it technically holds more water.
    • Typically this is a better rule of thumb to use when deciding how many fish to keep in your tank because it accounts for the oxygen needs of the fish, rather than just the available space in the aquarium.

Part 2
Determining What Kinds of Fish to House Together

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    Decide what sort of fish you want to keep. Do you want a community with lots of small fish? A semi-aggressive community with fewer, larger fish? Or maybe just one large fish? Look around, see what you like, then do as much research as you can on compatibility and maximum size.[5]
    • Some fish are more aggressive than others and need more space for themselves, while others do well in larger communities of fish. Do your research before you start integrating your fish.
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    Consider stocking levels. For small community fish (under three inches each), one inch of adult fish per gallon of water is a good guideline. For larger fish however, it can be tricky to decide how many fish. Assuming your list contains no fish too big for your tank or unsuitable to keep with other fish you want, start with one inch for every 7.5 liters of water and see how the tank looks.[6]
    • Don’t forget your fish will grow.
    • You want a balance between amount of fish on the top, middle, and bottom. This means considering where the types of fish you choose generally swim. For example, Plecostomus are typically bottom dwellers, while beta tend to stay at the top.[7]
  3. 3
    Research the specific needs of your fish. Always look into the individual characteristics of any fish you would like to put in your tank together. Many require different things to be satisfied in their tank.
    • Some fish are messy, some very aggressive, some nocturnal. Look at information from many sources to figure out these differences.
  4. 4
    Know the oxygen needs of your fish. Different kinds of fish have different oxygen requirements. Full-bodied fish like goldfish use more oxygen than slender-bodied fish like tetras. If you are stocking your tank with mostly larger fish, they will require more oxygen (and therefore, more space in the tank) than if you were filling the tank mainly with smaller fish.[8]
    • Stocking measurements should consider the adult size/weight of the fish in the tank to fully account for the necessary oxygen needs of the fish as they grow. So if you purchase your fish as juveniles, make sure you think about how big they will grow to become (and what oxygen levels the adult sized fish will require) when considering how many fish to keep in your tank.

Part 3
Maintaining Your Tank

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    Figure out filtration. You need more filtration for messy fish or large numbers of fish, but the largest filter you can get will always be helpful. Filtration systems help maintain the cleanliness of the tank and the purity of the water so your fish can thrive in their tank.[9]
    • Find a filter recommended for your tank size and get the one a size or two up, especially if you are pushing the limits with the number of fish you’re housing in your tank.
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    Maintain a consistent feeding schedule. Overfeeding your fish will lead to a messier, harder to maintain tank. This will eventually decrease the number of fish that can be safely sustained in your tank.[10]
    • Typically, you should feed your fish only what they can eat in 4-5 minutes. So, observe your fish during feeding time and experiment with different amounts of food. A good rule of thumb for smaller fish is three flakes of food per fish per day.
    • Overfeeding can also lead to high ammonia and nitrate levels, low oxygen levels, low pH levels, fin rot, improper digestion, mold, and cloudy water.
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    Clean your tank regularly. A dirty tank can cause a lot of problems for your fish. It can make your fish sick, uncomfortable, or even die. The dirtier your tank is, the less fish it can support because of compromised oxygen and filtration issues. You should generally clean your fish tank once a month, even if you have a good filtration system.[11]


  • Fish will not grow to the size of the tank, so when it says that 1" pleco will grow to be 24" – believe it!


  • Overstocking can lead to stunting, aggression, disease, and poor water quality. Avoid it at all costs.
  • Many fish grow very large; do not buy any fish that you don't know anything about.

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Categories: Aquariums