Fish to Water Ratio: How Many Fish per Gallon?

Last Updated on December 14, 2018

Many people often wonder how many fishes should be used to maintain an efficient and healthy aquaponics system.

When determining the capacity of an aquarium, it is common to look at total fish length or weight rather than the number of fish. The idea is that bigger fish need more room.

In this article, we’ll discuss the Fish stocking density in detail.

Fish Per Gallon Rule

For a smaller fish species, the most widely accepted stocking rule is to raise one inch of fish per gallon of water in the smaller tanks. This is not applicable to fish that tends to grow larger. As the fish grow, it increases in length, width and height. The volume and weight of 10-inches long fish can be more than 100 times of 1-inch long fish.

Hence, the best way to determine the stocking densities more accurately is in terms of pounds. A 6 -10 gallons per pound of fish is a fairly safe ratio in any type of system. The fish feeds according to their body weight. An adult fish can consume feed equals to the 46–68 percent of their body weight daily. The fish to feed conversion ratio (FCR) is also a popular parameter to determine the efficiency of a fish. Hence, larger fish tend to produce more waste and needs more space. It can be said that a 10-inch fish will eat and excretes a lot more than 10 x 1-inch fish.

These are rough estimates are to avoid any overstocking. You can increase your fish poundage to water ratio to get the most from the least. To increase the stocking density of fish, you will need more equipment such as air pumps is required to oxygenate your water tank. So essentially, this means higher costs on your behalf and need to monitor the system to keep it operating and maintained properly.

You can stock up to 1/2 pound of fish per gallon of water in larger fish tanks with proper filtration and aeration systems. However, this also depends on the species of fish. Some fish need more room than others.

Over-stocking of fish is one of the common mistake people make in aquaponics. It can lead to stunted growth for the fish and can even lead to the fish loss due to high nitrite levels.

You can also get a bit creative with the stocking density to see what suits your purpose the best. Let’s dive into details of stocking density and the ways of increasing it. At the end of the day, you want your fish to live in a comfortable environment.

Factors Effecting Stocking Density

You need to maintain the optimal minimum water quality requirements for your fish to ensure a comfortable environment for them. It depends upon the capacity of your system to handle waste vs. the capacity of your animals to produce it, and as the other answers say, there’s no simple fish:volume formula that Just Works. It depends on the following factors.

1. The Fish Type and Size

How many fish you keep in your tank largely depends on the type of fish you have chosen. Their temperament, adult size, and water quality requirements influence the possible fish stocking density.

Some fish like tilapia are hardy and can be stocked at higher densities. There might be fish that are fine for your tank in terms of bioload but are extremely territorial and wouldn’t have enough space to stay out of sight from each other. Some fish like trout needs high-quality water.

Read more about trout in aquaponics and tilapia aquaponics.

Some do best in schools and need a lot of open room to cruise in. Some fish live alone otherwise they fight each other (i.e. betta fish). Check more information about betta fish here.

You should also plan for the adult size and behavior. Otherwise, You may not want a red-tailed catfish or Arowana in a 10-gallon tank because it was only a few inches long at the pet store. Read more about the factors to consider while choosing fish for aquaponics here.

You also want to look out for combinations of species with such different water quality requirements that you can’t find a healthy point for both of them at the same time. Check the best fish to use in aquaponics here.

2. System Design and grow media

System design is very important in determining how many fish you can keep. There are 6 types of grow bed designs in aquaponics. All differ in the way they deliver the nutrients to plants. Some include growing media and some don’t.

Having growing media in the system allows for the bacteria to colonize on its surface. That’s why choosing the right growing media is very important. Read about the things you should consider while choosing aquaponics growing media here.

Growing media in the system acts as a mechanical filter to remove solid waste. With no growing media, 20L of gravel per fish is probably the maximum safe stocking density for fish such as Trout, Barramundi, Silver Perch etc, in an established system. Higher stocking densities could be achieved with mechanical filtration.

3. Water Quality

Water quality matters a lot in the capacity of the system to stock fish. The key to this all working is adequate DO (dissolved oxygen) in the water.

Fish waste conversion to plant nutrients requires ample DO, you should have a DO meter; and use it regularly, along with other water chemistry measurements.

DO Meter

pH is the main parameter of Water Quality. With an increase in the size of fish over the time, pH starts to drop below 7.0. the bacteria stopped reproducing and the ammonia levels started to climb. Water exchanges were an immediate fix but not a long-term solution. Adding in more bacteria and Potassium Hydroxide (KOH) to the system brought the pH back up to 7.3 and the ammonia levels soon went down to safe levels.

Season and Water temperature indirectly affects the fish density. As the sun crosses the equator, you may need to change the number of feedings per day.

In the spring and summer, the days are longer supporting up to 4 feedings every three hours, as verses 3 feedings in the fall and winter. This allows for more food and, therefore, waste in the system.

Couple this with warmer water temperature, and we have a faster fish growth rate as well as more nutrients available for the plants. The warmer water also contains less dissolved oxygen (DO), which is problematic.

4. Filtration Capacity

The filtration plays an important role in determining the aquarium capacity to support fish. A 10-gallon tank needs a 40-gph filter to run four times the total volume of water in the tank through the filter each hour.

In an aquaponics system, you are solely dependant on plants that acts a biofilter to recycle the waste. Your system must have enough capacity to absorb nutrients at the same rate they are produced. This is very crucial to ensure good water quality for fish. Some plants need a high amount of nutrition and others have a low nutritional requirement. Read more about Aquaponic fish to plant ratio here.

A system can be designed for either maximum fish growth or maximum vegetable growth, but not both. Keeping your fish water warm and removing fish solids is a method of maximizing fish growth and are some of the techniques used in aquaculture, as is adding pure oxygen to the water. However, warm water is often not conducive to good vegetable growth. These and other factors determine what it is that you will accomplish in your system.

6 ways to increase Fish Stocking density

Now you may have read about how some aquaponics systems can get more fish per gallon of water, that’s generally reserved for more advanced and complex systems. For a simpler system, especially if you’re a beginner, you should keep it simple.

1. Increase filtration

With an increased amount of bio-filtration (more grow beds), it may be possible to decrease the water to fish ratio to lower gallons of water per pound of fish. With a decrease of bio-filter volume, this ratio must be increased. This is based on feeding the fish as much as they will regularly eat and as often as is practical with no food left in the tank.

2. Decrease Fish Feeding

Another way to increase the fish density is to decrease the amount of food given to the fish. This will slow their growth, which may be desirable once they are fully grown. In any event, the real ratio here has to do with the amount of food digested and the size of the bio-filters needed to process the waste. As can be seen, the pounds of fish the system can support is more a function of the amount of bio-filter volume available than it is to fish tank size. This assumes proper design and selection of other system components.

3. Automate the FeedingAutomatic Fish Feeder

The best way to regulate and ensure that the fish are fed portioned amounts of food and on the schedule is to have an automatic fish feeder. The feeders we use have an eight-event per day timer and can be set to dispense food in one-second intervals. By setting the timer to dispense food for ten seconds, three to four times every day, which is about every three hours, we know that the food they receive is the same every day. This allows us to adjust the amount of food given the fish as needed and it also allows us to leave for a few days without worry that they won’t be fed. As the fish grow out, we increase the number of seconds on each feeding, thereby giving the fish more food. We highly recommend using an automatic fish feeder.

4. Keeping it clean

Another way to increase fish density is to remove the fish waste solids from the system. This would un-tax the system from the need of some of the DO (dissolved oxygen) in the water and would reduce the amount of system ammonia, but it would also reduce some of the resulting nitrates. In addition, this would remove some valuable plant nutrients that are a result of the mineralization process of fish solid waste. So, we leave in the fish solids, add heterotrophic bacteria weekly and provide ample aeration.

Through the process of solid waste mineralization, the nutrients and minerals found in the fish food make their way into the plants. So, what you feed your fish is what you feed your plants.

5. Increase DO

As your fish grew out and you have to increase the amount of food you gave them, the DO in the water decreased over time, which affected the nitrification process. DO is also a function of water temperature and this must be accounted for in making DO measurements. You may add aerators to your grow beds and also increase the aeration in the fish tanks. The DO then increase to a good level for the mineralization of the fish waste solids and the nitrification of the ammonia in the system.

As always, monitoring your water quality regularly, adjusting the amount of feed given the fish and adjusting your pH to maintain that quality is important.

6. Growing media and system design

It is recommended to design a system which provides maximum growth rate for both fish and plants.

Most optimal system design is flood and drain (ebb and flow design. The drain and flood cycles allow the bacteria on the surface of the Hydroton and aerate plants roots. Even with added grow bed aeration, which adds additional DO to the water, it does not reach all of the bacteria or vegetable roots as well as does draining and re-flooding.

Growing media you choose is very important. Hydroton clay pebbles are recommended.

Check the list of all Aquaponics growing media here


To summarize, increasing your fish density will produce more nitrates and needs more filtration, aeration, and space. In aquaponics, you can keep more plants to curb this problem. As the density increases, however, the system becomes less stable and requires more timely attention. This is true for a system of any size and type.

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