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Aquarium Size, Fish Stunting, sizing formulas

By Carl Strohmeyer-PAMR 40+ years experience
Updated 1/13/19

Determining The Maximum Aquarium Size For The Fish To Be Kept (including ALL factors, not just inches per gallon); As Well As Fish Stunting:

Quick "Jump" Index:

A lot of controversy surrounds the subject about tank size, often resulting well intentioned but incomplete advice being given.
This often a controversial subject among aquarists, especially well intentioned advanced aquarists, often resulting in confused aquarists (both beginner and even advanced).

One aspect is often missed and that is that although the gallons per fish rule is a good starting point, there is vastly more that goes into this subject (please see the section later in this article about aquarium sizing).
Also it is important to note that there is no denying that the bigger an aquarium you can afford, maintain or have space for the better for many good reasons!
BUT I have kept MANY aquariums under a variety of conditions and monitored them in controlled experiments and often a small aquarium can work for what many might consider over crowded conditions providing there is excellent filtration, cleaning maintenance, circulation, feeding procedures (and quality food), chemistry, etc.

For example you I can state categorically that 10 gallon aquarium with (2) 2 inch goldfish that is well maintained with a hang on the back (power) and a sponge filter will out have vastly better water parameters than a 20 gallon with the same goldfish that is poorly maintained with a corner bubbler filter.

Product Resources:
*AAP/SeaChem Tidal Premium HOB Filter
*Hydro-Sponge Patented Sponge filters

Similar to the points I have laid out in my “Aquarium Disease Prevention Article”, the more steps you follow beside the very basic amount of fish per gallon rule/suggestion, the more fish you can stock in a given aquarium.
Reference: Aquarium Disease Prevention; Steps for a Healthy Aquatic Environment

For example, I kept a 60 gallon Feeder Goldfish tank that often had over 500 goldfish at a time in it.
Is this over crowded?
Certainly, however I kept water parameters good by virtue of twice daily water changes, positive ion mineralization (such as calcium that is often ignored), large amounts of filtration. In the case of the aquarium I am speaking of, a larger tower fluidized filter along with a canister filter and air stones, and finally UV Sterilization, which I cannot say enough about.
This of coarse is an extreme example and NOT meant for long term fish keeping, only VERY short term. My point is I have kept similar Feeder tanks without the same procedures, filtration, etc. and the water parameters were NEVER as good.

Further References:
*Aquarium Chemistry; Depletion of Positive Mineral Ions
*About Aquarium and Pond UV Sterilization


As a more “Real World” example is all the controlled trials I performed (mostly using goldfish at the Bahooka Restaurant and Coaster Co. of America, which between the two clients I had over 140 aquariums).
Many of the trials involved filters and stocking and measuring the results and I often found better water parameters, and MUCH healthier fish in the smaller tanks that were maintained with the same number of fish as with the tanks that were often more than twice their size.

The difference was much better filtration, correct chemistry including GH & KH, better maintenance, and UV Sterilization.
All of these also have an affect on Aquarium Redox which is a major indicator of tank health as a healthy Redox Balance normally shifts a little between night and day, but an unhealthy Redox balance will often have large swings or have unusually high or low ORP numbers according to a Redox Neter.

*Aquarium Chemistry; Basic to Advanced
*Aquarium Redox
*Aquarium Redox ORP Meter

I should make one more point as per determining if a tank is too crowded or not (which was noted earlier about twice daily water changes), and that is the water change schedule. In fact if “flow through” water change methods are employed correctly, this can greatly increase the amount of fish the tank can hold (assuming you are not trying to keep a 12 inch arowana in a 20 gallon tanks where other issues then arise). This is possible in part due to improved Redox, mineralization and low hormone levels from the fish since these are removed via the constant changes of water.
See this article:
Aquarium Cleaning; Methods


It may seem like I am promoting situations that would induce stunting, but I am NOT.
I am referring to fish populations that can live comfortably within their surroundings and with their tank mates, or in the case of goldfish, knowing that anything less than an 8-10 gallon for short to medium term is not healthy, while a 20 gallon PLUS is much better for the long term.

Stunting would involve placing a baby Arowana or Quetzal Cichlid (or similar cichlid that will easily grow to 12 plus inches) in a 20 gallon aquarium.
Stunting IS CRUEL, and at the very least does not allow the fish to reach its full potential, including lifespan.

There are many theories that go into what happens biologically in stunting, here are a few thoughts:

Picture Examples of potentially over crowded aquariums
Both these pictures are photo-shopped examples, not actual aquariums.

vieja synspila Cichlid in small aquarium

In this photo-shopped example a Quetzal Cichlid (vieja synspila) is shown in a 14 gallon Bio Cube. This fish has NO chance of long term or even short term viability due to its large size, inability to move around, and inability to carry out its natural behaviors.
Honestly this picture needs little explanation.

Goldfish, Plecostomus in small aquarium

In this photo-shopped example, we have an aquarium that is a potential short to medium term problem and a definite long term problem.
The difference short term to medium term is filtration and maintenance. This is a viable tank while the goldfish are still young with good filtration/maintenance.

This tank shown has a built in wet/dry and separate Sponge Filter that has the pump directing water out to a 7 watt Terminator UV Sterilizer and back in.
If this tank was stocked with fish such as Platties that have no chance of out growing/stunting, this set up could easily handle a higher what is considered “Normal” number of fish.

Product Resource: AAP, Terminator Compact UV Sterilizers

Advantages for an Under Stocked Aquarium
Before it seems that I am advocating an over–crowded aquarium, I most certainly am not.

Here are a few benefits of an under stocked aquarium:


(Expanded information from “Freshwater Aquarium Basics”)

Start with as large an aquarium as you can afford and have space for (even for bettas).
The very BASIC principle that is to have 1-2 inches of NARROW bodied fish per FILTERED aquarium gallon is a starting point, but not very accurate.
This also only applies to a standard rectangular aquarium. Goldfish are dirty and fatter, so I would triple this with them, in fact for long term goldfish health, one goldfish per 8 gallons (30 liters) is best for long term health and "grow out".

Obviously longer fish need more tank width and length. I would decrease the amount of fish proportional to the gallons in a tall aquarium or hexagon aquarium.
Remember, many fish purchased can grow much larger than your original purchase size (ex: goldfish), so keep this in mind too.

To figure your tank size get your tank length, height, and width in inches then apply this formula (multiple all dimensions):
L x H x W = X;
Then divide X by 231
This gives you exact gallons of the tank.
In round tanks or unusual shapes you will have to extrapolate.
To convert gallons to liters multiply by 3.785
Example: a 20 gallon tank = 75 liters.

What is much more important in determining how many fish you should add to your aquarium are these factors:

Other Recommended Reference & Product Sites

Aquarium Fish Nutrition Facts
Fish Nutrition Facts; Fresh or Saltwater Aquarium

Freshwater Aquarium Care, fish sizing
Freshwater Aquarium Care

Sponge Filter Use Information
Sponge Filtration; Complete Sponge Filter Use Information

*Aquarium Silicone; USDA 100% Fish Safe
100% Fish Safe, USDA & Agricultre Canada approved.
The same CANNOT be said for Hardware Store brands!!

FSB Aquarium Filters
TMC Premium Fluidized Sand Bed Filters

Premium, second to NONE Aquarium Bio Filters, that with Oolitic Sand also maintain essential marine aquarium calcium levels, alkalinity, & electrolytes that are important to ALL Marine life, Goldfish, African Cichlids, Livebearers & more

SeaChem Prime

SeaChem Prime

Prime is the complete and concentrated conditioner for both fresh and salt water. Prime removes chlorine and chloramines. Prime converts ammonia into a safe non toxic form that is readily removed by the tanks bio filter. Prime can be used during tank cycling to alleviate ammonia/nitrite toxicity. Prime detoxifies nitrite and nitrate, allowing the bio filter to more efficiently remove them

Economy Submersible Aquarium, Fountain Pumps

Economy Submersible Aquarium, Fountain Pumps; SunSun JP-033

A better, UPDATED version of the Via Aqua 302 with SUPERIOR Performance, unlike other pumps sold elsewhere as a replacement

370 gph Aquarium Pump

SunSun HJ-1542 Aquarium Pump; replaces Via Aqua 1300

This pump replaces the Via Aqua 1300 and other copies such as by AquaTop as the Premier Power Head Pump for Aquariums, Ponds, Fountains, Wet/Dry Filters. This pump is submersible with Mag drive & ceramic shaft

TMC V2 RO Filter systems; the very best you can buy with TDS meter (far superior to 4 stage RO/DI systems sold via Bulk Reef Supply, Amazon, or eBay that use the inferior cellulose triacetate membrane made by Dow):

Reverse Osmosis Aquarium Water Filter, TDSReverse Osmosis Aquarium Water Filters; with TDS Meter


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In Chronological order of writing with the newest at the top

  1. How to Treat Sick Fish
  2. Whirling Disease in Fish
  3. Reef Aquarium Chemistry Maintenance
  4. Use of RO, DI, Softwater in Aquariums
  5. Lighting Theory of a Planted Aquarium- RQE, PFY, PAS, & PUR
  6. Aquarium or Pond Bio Load
  7. Tuberculosis in Fish
  8. PUR vs PAR in Aquarium Lighting
  9. Head Pressure in Aquarium and Pond Water Pumps
  10. Fin/Tail Rot For Betta & ALL Fish
  11. Angelfish Virus/Aids
  12. Activated Carbon
  13. Fish Baths/Dips as an aid to treatment
  14. Streptococcus gram positive bacterium in aquariums, Eye Infections
  15. Hydrogen Sulfide
    production in anaerobic De-Nitrification for Aquarium/Ponds
  16. Fish Shipping
  17. Aquarium Size, Fish Stunting
  18. Aquarium Algae,
    BBA & Brown Algae in particular
  19. Aquarium Salt (Sodium chloride) in Freshwater Aquariums
  20. Betta Habitat; Wild Bettas to Domestic Betta environment parameters
  21. HITH; Hole in the Head Disease
  22. Aquarium Protein Skimmers, Ozonizers
  23. Power Head/ Water Pump Review
  24. Molly Disease/ Mollies in an Aquarium
  25. Basic Fish Anatomy, Fin Identification
  26. Aquarium Moving/ Power Failures
  27. Octopus as Aquarium Pets
  28. Aquarium Nitrates
  29. Ichthyophonus protists, fungus in fish
  30. Aquarium and Pond Filter Media
    Types; Mechanical, Bio, Chemical
  31. Aquarium Water Conditioners (also Pond)
  32. Fish Parasites; Trematodes & Monogeneans; Annelids and Nematodes;
    Flukes, internal worms, Detritus Worms (often confused with Planaria), Micro Worms
  33. Aquarium Silicone Application;
    DIY Aquarium Repair & Glass thickness
  34. Pond Veggie Filters; DIY Bog Filter
  35. The difference between Plaster of Paris and Aquarium Products such a Wonder Shells:
    Identification, prevention & Treatment
  37. AQUARIUM TEST KITS; Use & Importance
  38. SEXING FISH; Basics
  39. Chocolate Chip, Knobby and Fromia Starfish
  40. Freshwater Velvet & Costia
  41. Usnic Acid as a Fish Remedy
  42. Aquarium Heaters; Types, information
  43. The Lateral Line in Fish, Lateral Line Disease
    or Head and Lateral Line Erosion (HLLE)
  44. Tap Water use in Aquarium; Chloramines, Chlorine
  45. Can Black Ghost Knife fish give an electric shock?
  46. Bio Wheel Review; Do Bio-Wheels really work?
  47. How do Fish Drink?
    Use of RO Water
  48. Cyclops, and Predatory Damselfly larvae
  49. Betta with Dropsy;
    Treatment and Prevention of DROPSY in all fish
  50. pH and KH problems in African Cichlid Aquarium
  51. Aquarium Gravel, which size?
  52. Blue green algae, Cyanobacteria in Ponds/Aquariums

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*Sponge Filters that far outperform all other brands or DIY:
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UV Replacement Bulbs/Lamps Directory:
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Fish Food Information:

Aquarium & Pond UV Sterilizer Use Articles
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