How Full Should an Aquarium be Filled With Water?

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Countless times I’ve been wondering how much I should fill my tank with water. Some people told me I should pour the water up to the brim. However, others encouraged me to leave a few inches from the top. To settle the argument, I decided to conduct extensive research on the topic. In fact, I spent over two days doing so.

Aquariums should be filled with water two inches to the top. That will allow ammonia, nitrates, nitrites, and other toxins to be diluted while preventing leaks. However, it is possible to fill fish tanks lower than that, as long as the foliage is entirely submerged and the fish can swim freely.

As we move forward in this article, I will elaborate on the advantages and drawbacks of filling the tank to the top. I will also discuss whether it is possible to overfill an aquarium and answer why the waterline can mysteriously go down over time.

How Much Water Should I Put in my Fish Tank?

Most aquarists understand the importance of maintaining good water quality. However, it never occurs to them to maintain the right water level. Those who have pondered this issue are unsure if there is a right water level, to begin with.

If you are new to aquariums and fish rearing and you are struggling to determine how full your tank should be, the following should add some clarity to your efforts:

1. The Right Water Level

Ultimately, there is no such thing as the right water level. The average expert expects every beginner to keep their fish in tanks of the right size. The objective is to prevent overcrowding and ensure that the fish have as much room as they desire to swim and explore.

Whether your tank is filled to the brim or the water level stops a few inches from the top, it doesn’t make a difference in the grand scheme of things so long as your fish have the freedom to swim. The amount of fish in the tank is more important.

So long as you endeavor to match the number of fish to the tank’s size (and the amount of water), the fish should be okay. Naturally, it makes little sense to fill a tank only halfway or even quarter-way; no one does this. 

Your entire aquatic environment should be submerged. That includes the foliage and decorations. That is why every aquarist endeavors to get as close to the top as they deem necessary. That being said, your decision to either reach the rim or fall a few inches short won’t affect the fish or the plants’ health.

2. The Benefits of Filling to the Top

As was noted above, your objective as an aquarist is to ensure that your tank’s components are submerged. On the whole, the water level doesn’t really matter. That being said, some aquarists will go out of their way to fill their tanks to the top because they believe it is more beneficial:

Maintenance – Larger tanks with more water are easier to maintain because they dilute toxins. Even if your tank is small, filling it to the brim gives you more water to work with. In turn, this makes the management of toxins like ammonia, nitrate, and nitrite easier. You have more room to experiment and to make mistakes.

Stocking – The more water you can add to a tank, the more fish you can keep. That is especially true for smaller fish. Every extra gallon of water creates room for a few more inches of fish.

Accuracy – Some aquarists believe that filling a tank to the brim is the only way to provide the room the tank guarantees. That is to say; every fish species has a minimum tank size that it requires. One example is the betta fish, which should be kept in a minimum of 5 gallons. 

When you buy a five-gallon tank, you can only offer your betta fish those five gallons of space by filling their tank to the brim. If you fall short, then your betta isn’t living in five gallons of water because you have failed to utilize the space provided by the tank adequately.

Appearance – Many aquarists prefer to fill their tanks to the brim because it eliminates the waterline, making it invisible and giving their aquarium a more seamless look. 

Can You Overfill a Fish Tank?

Fish tanks can be easily overfilled with water. While aquarists are usually encouraged to fill their tank with large water volumes, filling to the brim may result in a leakage. Instead, it would be better to leave two inches from the top.

Because there is no right water level for your aquarium, you are free to add as much water as you want. That being said, it is more than possible to overfill your tank:

1. How Can You Know that You Overfilled the Aquarium?

If the water is pouring from your aquarium, then you definitely overfilled it. That is the primary reason why some aquarists are reluctant to fill their tank to the brim. They are worried that the water will eventually spill from the edges, making a mess on the ground.

A tank filled to the brim will easily overflow whenever you stick your arm into it to perform regular maintenance. The same thing will happen whenever you add new items to the water, such as rocks and substrate. That is why some aquarists prefer to keep a half or a quarter of an inch below the top. That gives them room to maneuver.

2. Does Filling the Tank to the Brim Have Any Drawbacks?

Spills are not your only concern where overfilling is concerned. A tank filled to the brim will make it easier for fish to jump out of the water, as happens in betta fish.[1] Unfortunately, you cannot solve this problem by simply adding a cover. 

Unless it is porous, the cover will prevent the gaseous exchange between the air and the water surface. You can avoid this complication by keeping the water level below the rim of the tank. The exact amount of space you choose to leave doesn’t matter so long as all the tank components are submerged.

3. How Can You Determine the Right Water Level for Your Tank?

Interestingly enough, this question isn’t that difficult to answer for most aquarists these days. That is because the average tank comes with a black strip at the top, showing you the point at which the water should stop.

In most cases, this strip leaves an average space of two inches or less between the water level and the tank’s rim. If your tank doesn’t have this black strip, you can still use the measurements of other tanks that have the strip to approximate the right number of inches to leave at the top.

That strip highlights that the water in an aquarium should be high enough to keep the objects in the tank submerged. However, if your foliage grows low, it is more than okay to keep the waterline even beneath the black strip.

4. How do You Account for the Objects in the Tank?

Some amateur aquarists worry that they will displace water with the objects they add to the tank, eventually causing an overflow. However, this is rarely an issue. First of all, new tanks are set up in a manner that doesn’t permit the objects (rocks, driftwood, gravel, etc.) to complicate matters by causing an overflow.

When setting up a tank, I would recommend not filling the water all at once. Instead, it is best to fill it halfway. You may then add the plants and decorations, not to mention the heater, filters, and any other hardware you need. When everything is set, you can fill the water to the desired level.[2]

This process prevents the water from ruining the arrangement of the objects. It also prevents the objects from causing an overflow. That said, you are expected to account for the presence of objects in the water when determining the number of fish you can stock because, as was noted above, the objects displace water.

For instance, a 10-gallon will probably hold 8 gallons or less once you add all the decorations.[3] As such, you can only add fish in quantities that can fit in 8 gallons, not ten. That assumes that your decorations have only consumed 20 percent of the tank volume. The more decorations you have, the fewer gallons of water the fish have.

Why Does the Water Level in my Fish Tank go Down?

Water typically goes down in fish tanks due to evaporation. During this process, water molecules leave the tank and escape to the open air. That results in water levels that consistently drop. Overcoming the issue includes installing a sealed lid and reducing heat sources during the summer.

It is pretty prevalent for aquarists to complain about their water level, which mysteriously drops overtime. As was mentioned, evaporation is the most common reason for the issue. However, there is another option:

  • Leaks – Leakage should be among your first considerations. Start by checking your tank for cracks. It would help if you also looked at the filters because any cracks and faults they develop could cause the tank’s water to escape.
  • Evaporation – Evaporation is the primary cause of falling water levels in aquariums. Most aquarists expect their water levels to drop because evaporation happens all the time. However, with evaporation, the reduction in water is slow and subtle.

If your water levels are dropping drastically, you should look for a different cause because it can’t be evaporation.[4] It would be best if you also kept in mind that evaporation is complicated for two main reasons:

On the one hand, your fish will appreciate the process because it keeps the temperatures under control, lowering them during warm seasons. On the other hand, tank water isn’t pure. It is filled with toxins, minerals, metals, salt, and the like.

If you allow the water levels to drop via evaporation without taking steps to remedy the issue, these components’ concentration will skyrocket. This will change the chemistry of your tank, placing your fish in danger.[5]

Fortunately, it isn’t that difficult to control evaporation:[6]

  • Replacement – Regardless of the cause of the drop, you need to replace the water your tank has lost. Please take note of the water level by marking it on the tank. Whenever the water drops below the mark, add more. You can replenish the water you lost whenever you perform your weekly water changes.
  • Lid – This is the easiest and most straightforward solution. A lid won’t stop water from evaporating. However, the evaporated water will condense on the surface of the lid, eventually falling back.
  • Heat – It would be best to reduce the heat sources in the tank during warm seasons. Start by turning the heater down, eliminating some of the aquarium lights (or dim them), and keeping the tank away from windows.
  • Technology – Another choice is to buy a water level controller.[7] It will maintain the water level you have chosen by automatically refilling the tank whenever some water is lost. These devices can also prevent overflow. The WuyouChy Auto Top Off Water Filler (link to Amazon) is probably one of the most affordable choices in this section.

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Generally, it would be best not to fill aquariums to the top. That could potentially end up with a leaking tank and a huge mess. Instead, it is better to leave two inches from the brim. Fortunately, most aquariums these days feature a black strip that illustrates the desired waterline.

Rather than worrying about the water height, it would be better to focus on the fish, decorations, and foliage. It would be best to fill the tank in a way all its elements are fully submerged while leaving the fish plenty of place to swim.