how much evaporation is normal for a fish tank?

What To Do When Water Evaporates From A Fish Tank

(Last Updated On: May 23, 2021)

When water evaporates from a fish tank, it doesn’t get replaced. As a result, owners must regularly top their tanks up to prevent significant water drops that affect their fish’s health and welfare.

Most fish tanks experience at least ½ gallon of water loss a day. You can top up the tank a couple of times a week using clean, purified water, but don’t do this too often as you’ll risk affecting the water conditions. Instead, carry out a 10% water change every week or 25% every two weeks. To help reduce the chances of evaporation, keep the water as cool as possible and increase the room’s air humidity. Using a lid will also make a positive difference, as will turning off the aquarium lights as often as you can. 

Evaporation is a part of owning a fish tank, and not dealing with the problem causes a range of issues. Not only are the ammonia levels bound to rise, but evaporation decreases the amount of oxygen, putting your fish’s lives in danger.

Why Does Water Evaporate from A Fish Tank?

There are several reasons why water evaporates from a fish tank. A small amount of evaporation is normal and impossible to prevent entirely. However, it is possible to slow the process down if you’re aware of what causes it in the first place. These factors speed up the process of evaporation:

Warm Water

The biggest cause of evaporation is heat. Evaporation happens more quickly when the water temperature increases. That’s because heat causes water molecules to bounce around and collide, which moves faster as the water’s heat rises.

The molecules become energized, hitting each other harder and gaining more energy to transform from liquid to gas. At this point, the water evaporates from the tank.

Lack of Humidity

The lower the humidity of the air outside of the tank, the faster evaporation occurs. Dry air is further away from saturation, allowing a greater level of evaporation to occur.

Similarly, if the air is warm, it holds a higher water vapor concentration, leaving more room for water vapor to be stored. As you’d expect, evaporation occurs faster in the summer when it’s warm. Although, dry conditions are also responsible for fast evaporation, regardless of the season.

Air Movement

As described by Sciencing, the speed at which air flows over the water’s surface affects the rate at which evaporation occurs. That’s because air movement across the surface carries rising water vapor away. It also dries the air, making it easier for the water to evaporate. This is likely to occur when aquarium keepers use fans to cool the water surface.

Moving Water Surface

Water agitation speeds up evaporation. That’s because the ripples created across the water’s surface create a larger surface area, allowing the evaporation process to take place more quickly. Air stones and Gyre pumps flow more water across the tank and cause larger amounts of H2O to leave the aquarium.

fish tank evaporates quickly

How Much Evaporation Is Normal for A Fish Tank?

As we’ve already explained, there are a number of factors that determine how quickly your tank water evaporates.

Every aquarium’s evaporation levels are different because of the size, water conditions, and room temperature. Similarly, you’ll experience more evaporation in the summer than in the winter.

As a rule of thumb, a standard 50-gallon saltwater tank evaporates approximately ½ gallon of water per day. That means you’ll need to add 3.5 gallons of purified water every week.

Similarly, a 50-gallon freshwater aquarium loses around a gallon of water each day.

Saltwater tanks evaporate slightly more slowly than freshwater aquariums because the water molecules use more energy to break the dissolved salt ions apart in order for the water to evaporate.

How To Add More Water To A Fish Tank

Replacing evaporated water involves adding water to the tank. However, it’s a bit more complicated than this.

Simply topping off your aquarium after evaporation can be harmful to your fish. That’s because pollutants such as nitrate and phosphate accumulate. The water’s alkalinity and pH are also likely to drop. As a result, your fish will become stressed and more susceptible to diseases and infections.

To preserve your tank’s conditions, change 10% of the water in your aquarium every week or 25% every two weeks – whatever works best for your fish. A water change is beneficial because it:

  • Removes waste
  • Prevents unwelcome growths
  • Helps your filter

That being said, adding a touch more water to replace what’s evaporated every now and then is okay. To do so, using the following methods:


When topping up your tank water, only use pure, demineralized water to prevent harmful chemicals from getting into the tank and changing the conditions too much. This can put your fish under too much stress. If you don’t have access to pure water, you’ll need to make tap water safe before adding it to your tank.

Some owners pour purified water straight into the tank through a bucket or jug. Unfortunately, this usually leads to sudden changes within the tank, wreaking havoc on the aquarium’s ecosystem.

To safely add the water to your tank, store it at room temperature for a couple of days before pouring it in.


An easier way to top up your tank’s water levels is to use a semi-automated dosing pump. These devices are designed to pump small amounts of water into the tank, ensuring that the water levels don’t drop too low. The benefit of a dosing pump is that it prevents mistakes and stops you from forgetting to top up your tank.

How Far To Fill Up A Fish Tank

To work out how much water you need to add to your tank after evaporation, fill the aquarium up to your desired level and mark the glass with a pen or piece of tape. You can then use this to calculate how much water you need to replace every week. To do this:

  • After a week, measure the distance (in centimeters) between your current water level and the one you’ve marked up.
  • Multiply this value with the width and length of your aquarium.
  • The calculation will give you the amount of water that’s evaporated in one week in milliliters.
  • Divide this number by seven. This will give you your daily water loss number.

However, bear in mind that you’ll need to re-calculate this number when the seasons change, as you’ll lose more water in the summer when it’s warm and dry.

If you haven’t got time to work this calculation out or need a quick answer, subtract 1% of your water’s temperature to get a rough daily evaporation estimation.

How To Stop Aquarium Evaporation

You can’t stop aquarium evaporation completely, but you can slow the process down using the following methods:

Regulate Water Temperature

According to Environmental Science Investigation, fish can’t regulate their body temperature and rely on the water being at the right conditions to survive. Too much heat causes evaporation and also speeds up your fish’s metabolism, causing harmful toxins to rise in the tank.

While tropical fish species thrive in warmer temperatures, they’re able to tolerate a small heat range, meaning you can keep the water at the cooler end to prevent evaporation.

Fish also get stressed if the water’s too warm, so keep an eye on the temperature to ensure it doesn’t rise unexpectedly. This would not only increase the amount of water that evaporates but could negatively affect your fish.

Maintain Room Temperature

To prevent your tank water from evaporating too quickly, make sure the room you house your aquarium in remains as cool as possible without being too cold. This will prevent evaporation from occurring as quickly, as it’s harder for the process to take place in cooler temperatures.

Similarly, evaporation speeds up when the air’s dry, so maintaining humidity in the room should help prevent it.

As well as this, make sure your tank’s sensibly located away from sunny windows. If the room lets in too much light, place a cover over your fish tank for a couple of hours and consider placing the tank somewhere cooler and darker. 

Use A Lid

Adding a glass lid to your aquarium makes it harder for evaporation to occur. Similarly, the water that condenses on the inside of the lid drips back into the aquarium, slowing down the rate of evaporation. This emulates the natural process of rain dropping down to the ground from clouds.

Using a glass or plastic lid has other benefits, as it helps to maintain the water temperature and prevents fish from jumping out of the tank. A cover also prevents items from falling into the tank. Some tanks come with one, but they’re relatively affordable to buy separately.

Choose Your Fish Wisely

While it’s tempting to choose fish based on looks alone, this isn’t always the best way to create your community. If you live in a warm, dry climate, you’re better off choosing fish that prefer inhabiting cooler waters, such as goldfish. Other fish that can tolerate colder waters include:

  • Bloodfin tetra
  • Rosy barb
  • Buenos Aires tetra
  • Pearl danio
  • Zebra danio

These fish survive in temperatures around the mid-60s (Fahrenheit), meaning you can safely keep the outer environment at room temperature without affecting the fish.

Change The Water Filter

Fish filters are sometimes the cause of evaporation problems. Old, clogged-up filters will continue to try working, even if they’re malfunctioning. The filter will then get excessively hot and warm up the water. Not only will this cause evaporation, but it could burn your fish if they get too close.

If unclogging the filter doesn’t solve the problem, invest in a new, better one to see if your evaporation problem subsides.

Turn Off The Lights

Much like clogged filters, lights generate heat, especially if they’re cheap or old. As determined, excess heat causes evaporation.

At night, turn off all of your aquarium lights. As well as helping with your evaporation problem, this will enable your fish to get a good night’s sleep. Fish are diurnal, so they sleep at night when it’s dark like we do. Before you go to bed, switch off the aquarium lights and pop them back on again in the morning.

If your lights get excessively warm during the day, switch to high-efficiency, low-energy bulbs that provide enough light but don’t give off too much heat. If you use your lighting system to heat the tank, remember to turn down the heater to conserve electricity.

Test The Water

To prevent too much evaporation, regularly test the water temperature using a reliable thermometer to ensure it’s not rising too much. Sometimes the water temperature rises without warning, meaning it’s almost impossible to monitor by eye alone.

Some thermometers clip directly onto the tank, while others take the temperature from the inside. The most accurate thermometers are the ones that come with a probe, allowing you to test the actual water rather than the glass.

Either way, choose one that’s most likely to encourage you to stick to a regular temperature-taking routine.  

Reduce Your Tank Size

Wider tanks experience more evaporation because they have a larger surface area. If this is causing you a problem and your fish are suffering because you can’t keep on top of the tank’s dwindling water supply, you might be better off switching to a smaller tank and reducing the number of fish you have.

While this feels like a worst-case scenario solution, your fish will become sick if you allow harmful ammonia and nitrites to rise. Similarly, as we’ve already explained, simply topping up the water every day isn’t a viable long-term solution.

Dealing with evaporating water is a long, time-consuming process, so consider whether your tank is too large before deciding to change it.

Problems Caused by Water Tank Evaporation

While evaporation is part of owning a fish tank, it can cause significant problems that affect your fish’s health and wellbeing. Evaporation doesn’t always seem like a big deal, but it’s responsible for:

Rising Ammonia Levels

Evaporation leaves less water in the tank, increasing the amount of ammonia in the water. Evaporated water also leaves behind the nitrite and ammonia particles created by the fish’s waste. This isn’t too much of a problem in small quantities, but the tank’s conditions become unsanitary if the evaporated water isn’t replaced quickly enough.

Fish are sensitive to ammonia. If they’re exposed to it for too long, they become poisoned and burned by the toxicity. Signs of high ammonia levels include:

  • A foul odor that smells similar to urine
  • Fish gasping at the surface in the attempt to find air
  • Motionless fish who stop swimming
  • Appetite loss
  • Red or cloudy eyes from the burns
  • Redness around the fish’s gills

Regular water changes should prevent ammonia from becoming too much of a problem after the water’s evaporated.

Increased Salinity

In saltwater tanks, water evaporation leaves behind salts and minerals. As a result, the tank’s salinity increases, causing problems for your fish. Too much salt dehydrates fish and causes their scales to go slimy. In the worst cases, they die.

Automatic top-off systems are even more beneficial to saltwater tanks, as they keep the tank’s salinity levels consistent.

how to stop aquarium evaporation

Low Oxygen Levels

Evaporation lowers the tank’s oxygen levels. This is a problem for fish, as they require dissolved oxygen to breathe.

Unfortunately, it’s not always easy to tell that there’s not enough oxygen until it’s too late, but paying attention to the most common signs can help you keep an eye on the levels within the tank. Symptoms of not enough oxygen include:

As well as adding more water into the tank, you can increase oxygen levels by adding more plants, providing a suitable substrate, and decreasing the number of fish.

Increasing the aquarium’s surface area and aeration can also help, but these things are likely to speed up the rate that evaporation occurs.

Stressed Fish

Even the most minor environmental changes can stress your fish. If evaporation occurs too quickly and the water isn’t topped up, the fish will have less space to swim in. This creates overcrowding, and symptoms of stress will occur as a result. These include:

  • Increased aggression
  • Frantic swimming
  • Hiding away for long periods of time
  • Surfing the glass
  • Changing appearance, such as dimmer colors
  • Diseases and illnesses

Improper water conditions are one of the leading causes of stress. Even if one factor is off, your fish could suffer. As we’ve already explained, imperfect salt levels in a saltwater tank, rising ammonia, and changing pH levels are likely to enhance this stress.

Therefore, you must keep on top of your tank’s evaporation levels. By doing so, you’ll keep your fish happy and healthy. As we’ve already mentioned, evaporation is a normal part of tank ownership, so knowing what to do will help you provide the optimum conditions for your fish.