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How to avoid overstocking your fish tank

When it comes to stocking your aquarium, it is very tempting to add as much fish as possible. However, doing so will lead to water quality issues that will negatively affect the health of your aquatic pets. This guide will help you determine the right amount of fish to add to your tank.

What is bioload?

Bioload is the amount of excrements (i.e. poop and urine) fish produce in your aquarium. These organic wastes will eventually breakdown into ammonia. If you have a cycled tank, the beneficial bacteria will convert ammonia to nitrite and then nitrite to nitrate. Both ammonia and nitrite are very toxic to the fish. While nitrate is a lot safer, it can still be harmful in large quantities.

How to know if your fish tank is overstocked

You will know that your aquarium is overstocked when you test your water 24 hours after adding new fish. If your test kit is showing any presence of ammonia and/or nitrite, then it is very likely that you don’t have enough beneficial bacteria to feed off of the bioload.

What to do when your aquarium is overstocked

If you believed that your aquarium is overstocked with fish and is resulting in the spikes of ammonia, nitrites and/or nitrate, then you should immediately do a water change to bring those two compound down to safer levels. Make sure you also use a water conditioner, especially one that detoxifies ammonia and nitrite for a limited amount of time, for the entire volume of your tank (i.e. if you have a 55 gallon tank, use the recommend dose for that amount). We recommend using Seachem Prime, which you can find on Amazon with this link.

Continue to do daily water tests and changes until your tests show that you have 0 ppm of ammonia and nitrite for several days. This will most likely indicate that the beneficial bacteria colonies have grown big enough to handle all the bioload in your tank. To speed up the process, you might want to consider adding more beneficial bacteria through outside sources. If you are looking for a commercial product, then you should take a look at Seachem Stability on Amazon.

For a long term solution, you should consider adding live plants. Not only will plants consume ammonia and nitrate, they will also provide surface area for the beneficial bacteria to grow on.

What is the 1 Inch Per Gallon Rule?

The 1 Inch Per Gallon Rule states that you can add the as many fish as you want to your tank as long as the total length in inches does not exceed the same amount in gallons. In other words, you can add 5 inches of fish at maximum to a 5 gallon tank, 10 inches of fish to a 10 gallon tank and so on.

We have several issues with this rule. One of which is that different fish species have different bioload. For example, a Bristlenose Plecostomus, which grows to a maximum of 4 1/2 inches, will produce a ton of wastes. On the other hand, a Kuhli Loach, which grows to a max size of 4 inches, will produce a lot less in comparison.

The other problem is rule is that it is not indicative to how much space your fish needs. For example, a domesticated Koi will grow to at least 12 inches long. The 1 Inch Per Gallon Rule suggests that you will easily be able to keep this fish in a 55 gallon. However, the depth of a standard 55 gallon aquarium in just 12 inches from front to back. As a result, the Koi will have a hard time turning around if it is able to do so at all.

With that said, the 1 Inch Per Gallon Rule does provide a good starting point, particularly when it comes to stocking smaller fish. We just recommend that you don’t follow this rule blindly as you could potentially overstock your tank.

How to tell if your aquarium is properly stocked

In our opinion, the only way to tell if your aquarium is properly stocked with fish is to experiment. If your fish tank is fully cycled, then you should start adding fish to your tank. We recommend adding small amounts at a time just so you won’t overload your beneficial bacteria.

You should then use a test kit to measure the ammonia, nitrite and nitrate levels. A healthy environment should have 0 ppm of ammonia, 0 ppm of nitrite and less than 40 ppm of nitrate. You should worry most about ammonia and nitrite as small traces of both are extremely toxic to your fish.

If you are getting readings of the presence of ammonia and nitrite, then you should follow our guide above to make sure your tank can handle the added bioload. If, after a week of testing, you are consistently getting 0 ppm of ammonia and 0 ppm of nitrite, then you can introduce another small amount of new fish to your tank.

For nitrates, the fastest way to remove this compound in through a water change. Thus, you have somewhat of a balancing act as the more fish you add to your aquarium, the more water changes you have to do. As we mentioned previously, the goal is to keep your water parameters to less than 40 ppm.

If you want to do less water changes, then you should be more cautious when it comes to adding new fish. Of course, you can also add more fish as long as you are willing to do more water changes (and have stable readings of 0 ppm of ammonia and 0 ppm of nitrite).


There is no golden rule to follow when it comes to how much fish should you add to your fish tank.

Instead, we suggest adding a small amount at a time and then test your water parameters to make sure that the environment remains clean enough for the current inhabitants of the tank. After several days of getting consistent readings of 0 ppm of ammonia and 0 ppm of nitrite, then you can begin to add another small amount of new fish.

Nitrate should be kept to below 40 ppm. Assuming you can get constant readings of no traces of ammonia and nitrite, you can then correlate the amount of fish you want to stock to the amount of water changes you want to perform.

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