HOW MANY FISH PER GALLON? Aquarium Stock Guide

Last updated on December 24th, 2022 at 11:05 am

how many fish per gallon of water can we have in a tank? Before you experiment in the world of pet fish, one of the first questions that come to your mind is finding the right fish tank size. Tank sizes can vary depending on the fish you plan to catch.

Many fisheries experts state the rule of “one inch per gallon of fish.” Is this a hard and fast rule or a guide? 

A well-stocked fishery is truly a matter of beauty. How beautiful it is to see the beautiful fish swimming calmly among the carefully selected plants, rocks, corals, spines, and decorations! 

It needs careful planning to get this point. As a fish owner, you will spend a lot of time researching, investigating, and deciding what kind of fish you want to have, as well as what should grow best in your tank. It is a continuous process that will give you many hours of happiness.


What is the One Inch of Fish Per Gallon Rule?

In general, most hobbyists’ rule of thumb is 1 inch of fish per gallon of water. For example, if you have 2 inches size of fish, you need 2 gallons of water. However, this is a guideline as there may be exceptions to this rule, so it is important to understand what may affect these exceptions. 

You have to remember that an inch of fish per gallon rule is not the end of everything. That’s pretty simple. Wrong! The problem is that this overly simplified statement is almost impossible to apply to any new fish tank owner. This is because the fish we store in a fish tank are usually babies, and they will grow. 

I have never seen a pet owner in a fish store carefully calculate the mature quantity of fish he has purchased. This means trouble for some of them are on their way. 

In a general estimate, in a regular fish for 5 gallon tank of water, you can easily support 3-5 fishes if no one is fighters. The 10 gallon fish tank doubles it. 

Aquarium Stock Guide and size

Try to fill up your tank with low and high swimmers, and your fishery will look richer. You are keeping too many fish, and they have not equal space around the reef tank will cause a lot of unhappy creatures. They will fight over the same sand bed or rock structure. If you are trying to stick to this old rule, you need a better understanding of your fish’s preferred location in the environment. Doom works best if the fish swim all the time, but we do not create dioramas. We are creating saltwater aquariums.

Aquarium Fish Stock Guide

Fishes You Can KeepBody ShapeMax Length InchesHow many fish per 5 gallonHow many fish per 10 gallonHow many fish per 20 gallonHow many fish per 50 gallon
Fancy goldfishFull11.80002
Common goldfishFull13.80002
Common plecoFull15.70002
Clown loachFull15.70002
Sailfin pleco/gibbyFull17.70002
Bala shark/Silver sharkFull13.80013
Kissing gouramiFull9.80124
Sailfin mollyFull6.71225
Bristlenose PlecoFull5.11237
Boesemani RainbowfishSlender4.334614
Tiger barbSlender2.4581125
Zebra/Leopard danioSlender2691330
Ram cichlidSlender1.68121737
Neon tetraSlender1.211162250

Some Common Misconceptions About Fish Keeping Rule

Many aquarists think that the rule refers to the fishery’s current size when adding it to the list. But of course, the fish will grow! A fish about an inch long at the pet store can easily grow to six or eight inches when they reach adulthood – or more, depending on the species. You need to consider their maximum size as the fish mature. There can be a big difference in the fish sizes. 

One Inch of Fish Per Gallon Rule

As a fish grows, it does not become longer – it becomes taller and wider. Therefore, the space required for it is increasing exponentially. This is a hot topic in fish forums, where a lot of enthusiasts share their experience about whether you can use formulas to create the right ratio of fish to water. This is an easy mistake. You may have a 10-gallon aquarium size, but if you fill it with substrate, plants, rocks, ornaments, you have made a significant dent in the available water capacity. A 10-gallon water tank will hold only 8.5 – 9 gallons of water when fully installed. You can safely take the edge of the water 10-15% smaller than the actual tank size. Make sure you do your homework so you don’t deliberately add more fish than you can handle in your tank.

Aquarium Cycling

Before circulating your fish stocks, you must get the bacteria to begin the work to stabilize the water and prepare it for the fish. Here’s how: 

  1. Initially, after setting up your tank, let it run for a few days without the fish draining the water through the filter. Bacteria begin to establish themselves, including the good ones. 
  2. This is the time to get the temperature right, so you don’t have to experiment with your new heater at the expense of your fish. The 75-80 degree range is a good target, so choose in the middle of that range; you will delight more tropical fish. 
  3. Lastly, prepare a small fish meal, a small skimmer, a water test kit, and a siphon so you can be a responsible fish owner when the little ones arrive. I use a siphon that easily starts the water flow through the squeeze of an air pocket built into the pipe and prefers water test strips for ease of use.

Fish Aquarium Water Testing Kit

Now that you are on your way, start testing the water with the fish water test kit. You can find all types, but I use test strips, which seems to do a good job, and I like the convenience of using them. With the All-in-One Test Kit, you can track water for these key components: 

pH level: 

This is a measure of the acidity or alkalinity of your water. Your fish can take the outer limit, but they do not like rapid changes in it. The desired range is 6.5–8.2


It measures how well you can maintain a pH level in the tank. The optimum range for a freshwater tank is 120-180 ppm


Measures the amount of calcium in your water. Too soft or too hard the level can be dangerous to fish. The ideal range is between 50-150 ppm for almost all tropical fish. 

Nitrate and nitrite: 

These are made from the waste of bacteria that help to contain ammonia. It is better to have a level below 40 ppm for nitrate and 0.5 ppm for nitrite. 


It is made from the waste of your fish. In a new fish, it may be recorded at a bad level, but this number should not decrease once the good bacteria are gone. The ideal level for ammonia is zero. Start right now and measure your water for these things weekly and write it down. Then, you can monitor the progress and decide when it is safe to add some more fish.

Sizes Of Fish Tank

Fish tank dimensions should also be considered when looking at how many fish can be caught in a tank: especially large fish and fish with individual space needs. If you want to keep Oscar, pleco, or other large creatures, your fishpond should be at least half as wide as the mature fish, and more space is optimal.

  • 1-gallon aquarium dimensions are (8.5″ x 6.25″ x 9.25″).
  • 5-gallon tank dimensions are (16″ x 8″ x 10″).
  • 10-gallon fish tank dimensions are (20″ x 16″ x 12″).
  • 15-gallon aquarium dimensions are (24″ x 12″ x 12″). It is often recommended for medium size fishes like neon tetra and zebra.
  • 30-gallon tank dimensions are (36″ x 18″ x 12″). It is often recommended for medium to large-size fishes like goldfish and pleco.
  • 55-gallon tank dimensions (48″ x 12″ x 24″) are often recommended as a minimum for large fish. However, the 55-gallon community tank is only 12 inches wide, like the maximum amount of an adult Oscar fish or other big rusty cichlid.
  • 75-gallon aquarium dimensions (48 ″ x 18 ″ x 24) are very forgiving, which provides not only more volume for clean water but also a wider turning radius for your fish. 
  • 90-gallon aquarium dimensions are (48″ x 18″ x 24″).
Sizes Of Fish Tank

Fishes living in school and swim fast, break an “inch to the gallon rule.” 

  • Giant celestial danios fish grow up to 4 inches long and are nothing better than circling the fishery as a group. 
  • Silver dollars can reach 6-8 inches, and they love to stay in school.
  • The pleco can reach 24 inches and swim faster and tend to move as a group. These fish need to have long tanks that allow them to run back and forth naturally rather than demanding constant turns. 

Another concept that people do not consider is that your fishery’s actual size is less than you realize. 10-gallons of fish can be close to 8-9 gallons if filled with rocks, substrate, and vegetation. Not to mention the evaporative losses that inevitably occur. It is 10-20% less water to moderate chemistry changes, retain oxygen, and disperse structured CO2.

How To Keep Your Fish Tank Clean

Time needed: 2 hours.

After you have done fish keeping, check your water regularly and slowly add new small fishes, ensuring the tank runs well. Here are the follow-up tasks that should be on your calendar to keep the fish in the aquarium.

  1. Daily Tasks:

    Feed your fish twice a day, but only if they eat within 1-2 minutes. Ensure the temperature is still right, especially if the season’s change or the location is closer to a window. 

  2. Weekly Tasks:

    Run your water tests and write down the results. Use your siphon to remove 10% of the water from the tank and fill it with purified fresh water to remove the chlorine. 

  3. Monthly Tasks:

    Check your filter and replace it if necessary. Use your siphon to gravel vacuum to pick up loose debris. Replace polluted water with fresh water to remove chlorine. 

How To Keep Your Fish Tank Clean


It is important to know how many fish to put in your fish tank; the way you add them is even more significant. The 1 inch to 1-gallon rule is the old thumb rule, but it is not correct. The best way to measure is to test the water and add the fish slowly. Trust me, when you reach a limit, your water test will show it quickly, but it is better to avoid that moment. 


Remember, cycle your tank, add 2-3 fish to start, and test the water. Above all, be patient. You may want a fish tank full of fish but win the long, slow, and steady race. Do the number of fish because by taking careful care of your water, they should stay in your tank for a long time. This content is accurate and true to the best of my knowledge and is not intended to replace a qualified professional’s formal and personalized advice. Good luck! 

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