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How Many Fish Can You Keep in a Freshwater Aquarium: Guide

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Having a freshwater community tank is a great addition to any home.  However, ensuring that your tank isn’t overcrowded is crucial for the health of your fish. 

The standard rule of thumb is to have one or two gallons of water per every inch of fish. For example, guppies are about an inch long and you could technically have 20 guppies in a 20-gallon tank. However, if you have a larger fish such as a cichlid or oscar, they would need more space. 

Read on to learn more about how large of a fish tank you would need to accommodate your fish and other objects.

One Fish Per Gallon Rule

The one fish per gallon rule is widely known as the general rule when filling a tank. This general rule is that for one inch of fish, there should be one gallon.

This general rule will provide you an estimate of what you can fill your tank with, however, there are flaws to this general rule.

The one-inch per gallon does not take into consideration living plants, lighting, modern filter systems, or the type of fish that are in the tank.

Not all fish are the same size or even the same shape, this is the biggest fault the rule does not take into consideration.

For example in a ten-gallon tank putting ten inches of goldfish would not be the same as putting ten inches of guppies.

This is because the guppy fish have small slender bodies, whereas the goldfish are stockier and rounder.

Group of guppy and platy fish eating

Also smaller fish create less waste than larger fish such as an oscar of cichlids. Also more active fish need extra room to swim around in, even schooling fish.

For the 10 inches of guppies in a ten-gallon tank, this also becomes cramped as they are schooling fish and will swim together in the group. 

For schooling fish need to be kept in groups of their own kind. Some species require a larger group than others,  even though ten inches fills the ten-gallon, there simply may not be enough of them to avoid stress in your fish.

What about larger fish?

Larger fish create more waste than smaller fish do. When calculating how many fish you can fit in your tank you will also want to take into consideration the waste.

If you have a smaller tank filled with larger fish there is going to be much more waste. This excess waste can cause a number of problems, as well as constant cleaning

Too much waste can cause a spike in ammonia levels in your tank as there simply may not be enough beneficial bacteria in a smaller tank to control the ammonia levels caused by waste.

If you are choosing larger fish to fill a tank, it may be best to opt for a larger tank to lessen the maintenance or risk.

Room for Plants and Decorations

When following the one-inch per one-gallon rule, this follows the amount of water actually in your tank. A 10-gallon tank will not hold a full 10 gallons of water unless it is completely empty. Anything that is added to the tank will decrease the net volume of water. The more decorations in the tank, the less water there is and that means you will have a fewer amount of fish that can live in it comfortably.

This is a common problem more people do not realize as they do not take into consideration the loss of water volume.

This can result in overcrowding in smaller tanks as well as cause stress in the fish. This will also not allow room for your fish to live comfortably.

Live plants in the tank will take away from some of the water volume. However, plants also add oxygen to the tank water as it removes the nitrogen in the water.

Because of this the more plants you have, the more oxygen you have in your tank. This means with more oxygen in the tank, the more fish the aquarium can actually handle.

Filtration Systems 

Different filtration systems can actually determine how many fish can be put into the tank. This is because the filter systems need to be able to filter enough water to maintain stable water conditions and filter out wastes.

A filter should be able to run four times the total of water in the tank through the filter every hour. 

If the filtration system you have chosen does less than this, then it will not be able to support the one-inch-per-gallon rule.

There is never a danger of over-filtering the water in your tank. The better the filtration system in place the more fish it will be able to support in the tank.

You will want to choose the appropriate filtration system for your tank size that is powerful enough for your needs.

However some fish require different currents, some prefer higher currents, and some prefer little to no current. Because of this, it is best to choose a filter of the right size as well as output to suit your needs.

Time for Maintenance 

A very important factor when determining how many fish you want to stock your tank with is how often you will want to perform maintenance.

In order for fish to stay healthy and thrive they need to have certain water parameters. They need clean stable water in their tank environment to be happy.

The more frequently you maintain your tank, most importantly water changes, the healthier your tank will be.

If you do not want, or can not change your water frequently then you won’t be able to handle a lot of fish. The more on top of maintenance you are, the more fish you will be able to keep.

This is because when beneficial bacteria are unable to keep up with larger amounts of waste from multiple fish this affects the water parameters.

This can result in elevated ammonia or nitrite levels. This is essentially when you will pick up the slack for the tank environment.

Also, some fish foods can lead to more waste in your tank. Not all fish food is of the same quality. The lower-quality food actually contains fillers that the fish can not digest.

Because these fillers cannot be digested the fish causes more waste due to this.

A high-quality food will have fewer fillers in them. They are often referred to as clean foods, as with fewer fillers they result in less waste in the tank.

You will need to remove waste more often in your tank in order for the beneficial bacteria to keep up with the waste. By removing polluted water and replacing it with fresh clean water to dilute the pollutants. 

Depending on how often and how well you maintain your tank can also be a determining factor when stocking a tank.

Prior to calculating with the one-inch one-gallon rule, you will need to figure out how much you can maintain the tank.

This can also help you determine the kind of fish you will want in your aquarium depending on their needs and care levels.

It may also be best to get fewer fish at first to see what you can maintain sufficiently, as you can always add more tank mates later on!

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