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1.1.8. Stocking

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Author : David Bogert

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5 minutes
The number of fish and the size of the fish in an aquarium is called the “stocking” of an aquarium. Any beginner to the hobby should start with a few fish under three inches (7.5 cm) in size and gradually increase either the size or the number of fish in the aquarium over the span of at least a few months. Adding too many fish or too large of fish too soon can overload the filter and the aeration of the aquarium and kill the fish.

If one is new to fishkeeping, stocking is best looked at not by numbers but rather by water clarity. Slowly add some fish. Then see how it goes. Keep looking down the length of the tank and judging how clear the water is. If the water looks even a little bit cloudy do not add any more fish till the cloudiness clears up. Don’t worry about numbers. Gage the stocking by the health of the tank. How clear is the water? When the water is clear, add more fish. Cloudy water? Wait a while, till the cloudiness or “dullness” clears. Then you can add more fish. Simple!

Note that it is little “science” involved in the whole topic of stocking. There is a series of journal papers on stocking of zebrafish which support VERY heavy stocking, two to five times the heaviest stocking the author recommends and six to fifteen times the stocking levels at the maximum levels recommended by web stocking calculators. Other than those papers the whole topic amounts to nothing more than one OPINION after another OPINION. And the author’s OPINIONS are no better than anyone else’s OPINION on the topic.

Trigonostigma heteromorpha Harlequin Rasbora
Trigonostigma heteromorpha – Harlequin Rasbora

Stocking in More Depth

The basic stocking rule that has prevailed forever is “one inch of fish for every gallon of water” or “one centimeter of fish for every liter of water”. This is a decent gross approximation for beginners with their first aquarium and fish that are one to two inches (2.5 to 5 cm) long. The “rule” breaks down rapidly above two inches (5 cm) and for more advanced hobbyists. The beautiful Lake Malawi African cichlid peacock aquarium below shows how one can put hundreds of four to five-inch (10 to 12.5 cm) fish into a 100-gallon (379 liter) tank. This is at least five times the “one inch of fish for every gallon of water” level. And the water in this tank is crystal clear, indicating it is not overstocked and well filtered.

Very heavily stocked aquarium
Very heavily stocked aquarium

More Accurate Stocking

For fish, more than three inches (7.5 cm) or for a more accurate assessment of stocking one has to look at volume, weight, and metabolism rather than a “one inch per gallon” (one centimeter per liter) standard. If one looks at the volume, weight, and metabolism of the fish, not solely the length, one gets the following tables:

For a 100 gallon (378,5 liter) tank the stocking numbers are:

Total length of fishLight stockingRecommended 100% level stockingHeavy stocking *
1in 2,5cm1816301890
2in 5cm2893280
3in 7,6cm103297
4in 10cm51750
5in 12,7cm31133
6in 15,3cm3927
8in 20,3cm1411
10in 25,4cm126
12in 30cm013
* ONLY with very heavy filtration
Number of fish for stocking in a 100 gallon or 378,54 liters aquarium

For a 50 gallon (190 liter) tank the stocking numbers are:

Total length of fish in centimetersLight stockingRecommended 100% level stockingHeavy stocking *
1in 2,5cm90315945
2in 5cm1447140
3in 7,6cm51648
4in 10cm2825
5in 12,7cm1516
6in 15,3cm1413
8in 20,3cm025
10in 25,4cm013
12in 30cm001
* ONLY with very heavy filtration
Number of fish for stocking in a 50 gallon or 189,27 liters aquarium
Balantiocheilos melanopterus, Bala Sharks
Balantiocheilos melanopterus, Bala Sharks

For a 25 gallon (95 liter) the stocking numbers are:

Total length of fish in centimetersLight stockingRecommended 100% level stockingHeavy stocking *
1in 2,5cm45159492
2in 5cm62390
3in 7,6cm2824
4in 10cm1412
5in 12,7cm038
6in 15,3cm027
8in 20,3cm013
10in 25,4cm001
* ONLY with very heavy filtration
Number of fish for stocking in a 25-gallon or 94,64 liters aquarium
Hyphessobrycon erythrostigma Bleeding Heart Tetra
Hyphessobrycon erythrostigma – Bleeding Heart Tetra

For a ten-gallon (38 liter) the numbers are:

Total length of fish in centimetersLight stockingRecommended 100% level stockingHeavy stocking *
1in 2,5cm1863189
2in 5cm3928
3in 7,6cm139
4in 10c015
5in 12,7cm013
6in 15,3cm012
8in 20,3cm001
* ONLY with very heavy filtration
Number of fish for stocking in a 10 gallon or 37,85 liters aquarium
Corydora agassizii
Corydora agassizii

For a five-gallon (19 liter) the numbers are:

Total length of fishLight stockingRecommended 100% level stockingHeavy stocking *
1in 2,5cm93190
2in 5cm1414
3in 7,6cm014
4in 10cm002
* ONLY with very heavy filtration
Number of fish for stocking in a 5 gallon or 18,93 liters aquarium

These numbers simply reflect my OPINION as to stocking. There is little “science” involved, These numbers will send some self-appointed social media “experts” (i.e. the “fish police”) into absolute hysteria and get us kicked off many Facebook forums. But, in my OPINION, even the heavy numbers are easily achievable with no negative impact on the fish, SO LONG AS FILTRATION AND AERATION ARE ADEQUATE.

Aphyocharax paraguayensis Dawn Tetra
Aphyocharax paraguayensis – Dawn Tetra

Look out with the heavy stocking columns for the ten and five gallon (38 and 19 liter) aquariums. The filters for small tanks are always very small cartridges or air-operated sponge filters. These filters are just totally inadequate to support heavy stocking.

So small aquariums should not be heavily stocked. If one puts something like a canister filter on a small aquarium, one can think about heavy stocking. But I’ve never seen that done.

Note that the goldfish is very controversial. Common or “narrow-bodied” goldfish can be considered five-inch (12.5 cm) fish for stocking purposes. So-called “fancy” goldfish with short spines and fat bellies are different and need to be looked at as six-inch (15 cm) fish.

But there is a concerted campaign by PETA to make sure every goldfish in the world has at least thirty gallons (100 liter) of water per fish to swim in. The PETA folks are well-meaning but they are equating a fish with a person. This “anthropomorphism”, gives an animal the characteristics and emotions of a human. This simply isn’t true.

Goldfish are analyzed in more depth in this article:

17.5. Goldfish

Corydora Julii
Corydora Julii

When starting one should only stock at the light stocking level. Then in three or four months, one can move up to moderate stocking. And in six months, the aquarium CAN go into a heavy stocking. Note the emphasis on “can”. A beginner should take things slow.

All this is predicated on the premise that the hobbyist is not cleaning the filter media regularly and is allowing a goodly amount of beneficial brown gunk to build up in the filter media. Thorough cleaning of filter media is NOT a good thing to do. The brown gunk is a beneficial bacteria. “Beneficial bacteria ain’t pretty”.

Amatitlania nigrofasciata Convict
Amatitlania nigrofasciata Convict

Myths about Stocking

Like everything in the aquarium hobby, there are a large number of myths parroted around social media about stocking.

Some of the common myths parroted around social media about stocking, in my OPINION, include:

  • Heavy stocking of a fish aquarium will shorten the life of a fish and is “cruel”.
  • A fish can appear healthy and fine but not be “thriving” or “happy”.
  • Each fish has a minimum aquarium size and length beyond being large enough to swim freely in.
  • Fish in a small aquarium will not grow to their full size.
  • Have one gallon of water for every inch of fish. (One liter water for every centimeter of fish)

These myths are all simply false, in my OPINION.

Dawkinsia filamentosa Filament barb
Dawkinsia filamentosa – Filament barb

To go into a stocking in far more depth click on this link:

13. Stocking