How to Change Fish Tank Water (Without killing your fish)

Even when your water is not cloudy, it can still need cleaned.  If you stir up your substrate, you are likely to find the water suddenly cloudy, showing that the tank and water need cleaning.  However, everyone has heard of that person that cleaned their tank and then all their fish died.  It is important that you clean the tank correctly to avoid killing your fish.

How do you change fish tank water without killing your fish?  Changing the water should be done accurately and routinely to protect the health of your fish.  Without cleaning, the water becomes dirty and depleted of needed minerals.  However, when cleaned incorrectly, the shock of new water can kill your fish.

Particles of food fall the bottom of the tank during each feeding.  Fish urine and feces are released back into the water.  Even the best filter cannot keep the water from building up waste.  The dirty water can stress the fish and lead to many health problems.  However, it is important when setting up a routine to consider the type of change, number of fish, and kind of fish to make sure they are not harmed.

Reasons for Changing the Water

Most places fish live in nature have very low nitrates in the water because the water is constantly being “changed” through moving downstream or spreading throughout a large body of water on currents.  However, when fish are kept in a tank, the water cannot flush itself clean.  This results in increased nitrates and phosphates, changes in the pH level, and lead to toxins in the water that can harm your fish.

Nitrates and Phosphate

Elevated nitrates will stunt the growth of you fish and can interfere with adult fish reproduction.  A high nitrates level promotes overgrowth of algae, which impacts water quality for the fish.  Phosphates will have a similar impact on the tank.  Nitrates and phosphates are only removed by changing the water.


Uneaten fish food, dead particles, and excrement often sink to the bottom of the tank and end up in the substrate where they cannot be filtered out.  The water may appear clean because they are trapped in the substrate, but they are still impacting the water quality.  The rotting particles can give off toxins that are highly dangerous to your fish.

Trace Elements

All bodies of water have trace elements and minerals that the fish living in them need to survive.  These elements are replaced through rainfall and soil run off.  In a closed fish tank system, the elements cannot be replaced.  Others are filtered out.  The lack of these trace minerals will decrease the vigor and health of the fish.  Giving them new water replaces the minerals.

The nitrates, wastes, and removal of trace elements over time will impact the pH level in the tank.  An improper pH level is unhealthy for fish and will have to be changed up or down to reach the optimal level for the fish in the tank.  Switching out the all or part of the tank’s water will allow control over the level of debris and toxins.  It also allows the monitoring of the pH level to prevent problems.

Schedule of For Changing the Water

Water changes should be part of tank maintenance.  Some people change their tank weekly, some monthly.  The correct schedule will vary based on many factors.  The size of the tank, the number of fish, and how much food is fed all impact how often the tank needs changed.  Testing the water for nitrates is one way to determine when the water needs changed.

In most moderately stocked tanks, the nitrates level will rise weekly.  Nitrates need to be kept below 40ppm.  By testing them, it can be predicted when the water in the tank will need to be changed.  A tank produces 10ppm of nitrates each week and will reach 40 ppm after four weeks.  However, if a partial change is done, remember this will not reduce the nitrates level to zero.  So, the next change will happen sooner.

How often a water change is needed can be reduced by having less fish and by feeding less.  A larger aquarium also decreases the time between water changes.  The more volume of water the more the waste is spread out in the water, lowering the parts per million.  Adding live plants can also decrease the number of changes as they eat nitrates.  These tips will not eliminate, only decrease, water changes.

Types of Water Changes

When changing the water in the tank, it is possible to change part of the water, all the water, or just top off.  With the partial change, only some of the water is removed and new water put in to replace it.  What percentage of your water depends on your desired frequency of changing the water and how many fish are in the tank.  Typically:

  • Average Aquarium: 10-15% water
  • Heavily Stocked: 20-30% of the water
  • Lightly Stocked: 10-15% of the water

Partial Water Change

The amount of water changed will depend on how often you change the water and how heavily stocked the tank is.  Typically, 10-20% of the water is changed for a lightly stocked tank.  For a more fully stocked tank, 20-30% of the water is usually changed.  Some people will change up to 50% of the water, but this should be considered typical.  Monitoring nitrates will help determine how much is changed.

Remember the amount of water changed, determines how much the nitrate level is decreased.  It is important to the nitrates lower enough that after one week, they will still be below 40 ppm.  For example, if you only change the 10% of the water, this will only lower the level to 36 ppm and you may need to change the water more than once weekly if the fish tank rises more than 4 ppm in a week.

For heavily stocked tanks, a 30% water change may be needed to keep partial cleanings weekly.  Larger water changes can increase the time between cleanings, but the larger the change, the more stress on the fish.  Water changes are done to keep the fish healthy.  By changing too much water, it is possible to contribute to increased risk of stress and illness.

Complete Water Change

A complete change requires removing the fish from the tank.  This is very stressful to the fish and can lead them susceptible to illness.  Complete water changes should be reserved for moving the tank, a large build-up of ammonia, or for a containment in the tank.  Even when partial changes have been infrequent, a complete change is rarely needed to “catch up”.  They may do more harm than good.

Topping Off

Fish tanks lose water to evaporation and other sources.  Over time, the water level will drop.  This lost water can be replaced during water changes.  However, additional water can be added, when needed, between changes.  This procedure is called topping off.  Although it does add water, it does not remove any of the toxins or debris and so, should not be used instead of or to delay partial water changes.

If there is significant evaporation between water changes, the changes are too far apart, or there is something wrong with the set up.  An aquarium properly set up should have very little evaporation between cleanings.

Preparing the Tank

Prior to changing the water, you need to pretreat the water you will be adding and prepare the fish tank for the new water.  Most tap water has been treated to make it safe for human consumption.  However, these treatments can make the water unhealthy for fish.  Pretreating the water removes the dangerous chemicals and metal residues.

Treating the Water

You can purchase water treatments and conditioners anywhere fish are sold.  It is important to follow the directions for the specific conditioner you buy.   The water conditioner is based on your type of tank and the fish in it.  Some are available based on tap water problems, such as hard water.  If you are unsure which to use, as for recommends where the fish were purchased.

Using a clean bucket, get water from the tap, and let it run for five minutes.  Following the directions on the water conditioner bottle, pretreat the water.  Most conditioners are just poured into the water and allowed a specific time period to work.  The bucket used needs to be free from soap and dirt residue. 

It is important to do this step prior to siphoning out the water.  Many conditioners need time to work, and the treated water needs to sit for a period of time before use.  Allowing it to sit for one day will also allow gases to dissipate and the pH level to stabilize.  It is important to try and get the water to room temperature or as close to the tank’s water temperature as possible.

Tank Fixtures

Before you get started, you should unplug the tank’s lighting and heating elements.  This will decrease electricity while working around water and decrease the risk of an electric shock.  Most filters do not do well when the water level drops. So, it is necessary to unplug them prior to water removal.  At this time, you can inspect the filter to see if it needs cleaned or replaced.

Most filters will not need to be replaced every time you change the water.  In fact, replacing the filter too often can remove too much of the good bacteria from the tank.  Follow the manufacturer’s directions or look for obvious signs that it needs replaced during a water change.  During most water changes, the filter needs rinsed in cold water. 

Tank decorations can be inspected to determine which if any need removed and cleaned.  It is not necessary to remove all decorations for every water change.  Just with the filter, this will remove too much good bacteria.  However, any that look slimy or sludgy will need to be removed and allowed to soak in a cleaning solution.

  • Never wash your plants and decorations in soap
  • Soap residue is harmful to fish and will increase algae in the tank
  • Soak your plants and decorations in chlorine bleach and water mixture
    • 1-2 tablespoons of bleach per bucket of water

Lastly, the walls of the tank will potentially need to be washed.  At every water change inspect the tank walls.  Any green or brown film on the sides of the tank will need to be removed using an algae sponge or scraper.  Gently rub to remove residue prior to removing water.  If you are cleaning your tank regularly, this doesn’t need to be done every time.

Removing the Water

An automatic water changer is the most efficient way to remove the water, especially with larger tanks.  These work by creating a siphon that sucks the water out of the tank and into the sink until it is shut off.  It minimizes the risk of spilled water and eliminates carrying buckets of water around.  For smaller tanks, or to do it manually, a siphon can be used.  Leave the fish in the tank during this process.

Make sure to siphon out any substrate debris with a gravel siphon.  Place the siphon tip into the tank’s substrate at an angle.  It is best to section off the tank and only clean certain areas of gravel each time.  This decreases the impact on the fish.  Not all the gravel will be cleaned every time.  When large sections of gravel are cleaned, it is better to clean the filter on another day to preserve the good bacteria.

Remember to only take 10-30% of the water at one time. Going beyond this can stress the fish and damage the balance of the tank.   A good way to do this is by using your bucket size to measure the amount of water being taken out.  For example, a 3-gallon bucket would hold 30% of a 10-gallon tank.  This way, you keep an accurate measure and do not remove too much.


Note the temperature of the water following the water drain.  Make sure the clean fresh water is the same temperature.  If not, allow both to continue to sit and reach room temperature and match.  Water temperature fluctuations are harmful to fish.  After adding the fresh water, check the thermometer a final time. 

Refilling the Tank

Now that the tank is cleaned, it is time to put the water back in.  You can empty the pre-treated water already prepared slowly from the bucket into the tank.  Or, if the bucket is too large to pour slowly, use pitchers to take water out of the bucket and pour into the tank.  The water needs to enter the tank slowly so that it does not disturb gravel and decorations. 

  • Replace the cleaned decorations and plants.
    • They can be placed back in the same places or moved around. 
  • Reconnect and restart the filtration system, heater, and lights. 
    • Some filters will need 1-2 cups of water directly fed into the system before they will start working. 
  • Finally rinse and store the cleaning equipment. 
    • The buckets, scrubbers, and siphon need to air dry before storing
    • Not using them for anything else will make sure they do not get contaminated and pass that along to water entering the tank.

Long Term Maintenance

Fish tanks need cleaned routinely.  Knowing your nitrate growth rate can help you plan on weekly, bi-weekly, or monthly cleanings of your tank.  These changes need to happen on a consistent basis because when the toxins, nitrates, and tank chemistry become unbalanced, it can be very difficult to get re-balance the tank.  Each cleaning will only remover 10-30% of the water. 

“Reset” your tank with an unscheduled water change when needed.  But, don’t try and keep it “too clean” either.  Let fish health and nitrate level drive the frequency.  Too frequent cleanings can adversely impact fish health as much as too few. 

Limit light usage.  The light will help algae grow.  Keep lights on for 6-10 hours unless you have live plants.  With live plants, use the lights for 10-14 hours.  If you are having high amounts of algae, try the lowest recommended setting for your tank to see if less light will decrease the algae.

Overfeeding is not healthy for the fish.  Much of the debris in the gravel is leftover food.  Stick to once or twice a day feedings.  Watch how much the fish eat.  After the swim away if large amounts of food remain or fall the bottom, decrease slowly the amount fed.  If some of the fish eat at the bottom and do not rise to the surface, some feed falling is okay, but still needs to be kept limited to avoid the need for more water changes, which is not recommended.

Some Common Problems

Even when a fish tank is cleaned routinely and correctly, problems can occur.  Finding the cause and determining the solution quickly can be key to fixing the tank prior to fish death.  Most of them ultimately come down to water quality problems.  Learning about them can help prevent them and treat them.

Cloudy or Green Water

This typically happens in new tanks and can be the result of a few different causes.

  • Fine Particulate: In new tanks, fine particles in the substrate can cloud the water.  They will filter out in a few days.  This is why it is often advised to set up and run a tank for a few days to a week prior to placing the fish.
  • Dissolved Minerals:  Cloudiness can result if the source water is high in dissolved minerals. This can be checked through the pH level.  Finding a purer source of water or using a water conditioner can help. is another option. Change the pH slowly if fish are already in the tank.
  • Bacterial Bloom:  A bacterial bloom is fairly common in a new tank but can occur at any time. Typically, the culprit is some source of nutrients from overfeeding, decaying plants or fish, or a general lack of maintenance. The key is to remove the bacteria’s food source through reduced feeding and/or water changes.
  • Green Water: Green water is due to free floating algae growth.  This is caused by and treated similar to a bacteria bloom.  One additional consideration is cutting back on the lighting for the tank.  Algae can’t grow without enough light.

Ammonia Spikes

Ammonia can be a problem in a new tank prior to a bacterial colony is established.  It can also be a problem in an established tank because of a loss of bacteria or a new source of ammonia.  Most excess ammonia comes from something rotting in the tank, such as a dead fish, plant, or a clogged filter.  It can be caused by excessive waste in the gravel. 

Too much cleaning can remove the good bacteria through removal of the surfaces they grow on.  This can include the gravel, decorations, and filter.  It is important to only clean parts of these when doing a water change to allow the good bacteria to continue to grow.

The goal is to manage the ammonia until the bacteria can adjust and decrease the level.  A tank with living fish will need water changes and close monitoring.    Feeding the fish temporarily may be necessary to decrease the stress on the system.  Aeration of the water can also help dissipate the ammonia.

pH Problems

It is almost more important to keep the pH level stable than trying to match your tank to the exact level recommended for your fish.  If they seem health and the pH is not changing, then adjusting the pH level is not recommended.  However, if the pH level is too far outside the desired range, then a few methods can help adjust it.  It is very important to go slow with a tank with live fish.

The following can be added to the filter a little at the time to change the pH in small increments.  Add a small amount and then wait for it to stabilize before adding more.

  • Lower pH – Peat Moss
  • Raise pH – Crushed coral

If the source water is a problem, adding distilled water to it (which has a pH of 7) may help bring it up or down before you use it.  Commercial products for raising and lowering pH should be avoided if possible.  They are a quick fix and will not change the pH in the tank long-term so you would have to constantly keep adding them.

Oily Film on Water Surface

Fish food can be the number one source of introducing oil to the tank water.  They contain fats and oils.  Oils from your skin, lotion, or any other skin care products can left behind during maintenance.  Oily droplets in the air from cooking can also drift into the room and be deposited on the water surface.  Damaged tank equipment can also be a source.

Whatever the source, the oil is not likely to be harmful (other than the skin care products).  But, it can interfere with the gas exchange at the water’s surface throwing off the tank balance and resulting in less oxygen for the fish to breathe.  The more oil on the surface, the less area for the oxygen to move into the water.

The first step is to determine the source of the oil.  To remove the oily film in the water, turn off the filter and anything else that will cause turbulence.  Lay a paper towel gently on the surface of the water to soak up the oil.  Also, the top of the water can be skimmed during the next water change to remove some of the water.

High Nitrates

  1. nitrogen cycle that has been set up in the tank.  That does not mean they are good, just part of the process that keeps the tank healthy.  Bacteria can help consume the nitrates can change them to nitrogen.  But, those bacteria are difficult to culture and grown in a home tank.

A theme of many of the problems is water changes.  Water changes can help prevent or treat many common problems.  Regular maintenance can prevent the problems from happening.  An unscheduled cleaning can treat problems that have happened.  Even when things seem to be going well, make sure and do a few simple tests to monitor water quality and catch problems before they are severe.

Final Thoughts

Changing the water in a fish tank can seem like a daunting task. We have all heard horror stories of people who changed their water just to have all their fish die soon after.  This can be due to many different reasons, but it is often caused by waiting too long between cleanings or cleaning the tank incorrectly.  Fish are easily stressed which can increase their likelihood of disease or death.

But, with some pre-planning and knowledge, changing the water completes some needed maintenance that keeps the fish healthy and can be done without killing all the fish.  It is important to use proper equipment, water conditioner, and to only partially remove the water.  The fish need to be left in the tank while it is being cleaned to reduce their stress.

Care needs to be taken when putting back in fresh water that is has been properly treated, is the right temperature, and that it is poured back into tank carefully and slowly.   This will keep the fish from being shocked by the new water and becoming susceptible to disease.  Proper filter maintenance can be done at the time of cleaning.  These directions will allow you to change the water without killing your fish.