Why do Fish Swim at the Top After Water Change?

After a water change, fish may swim at the top due to a lack of oxygen. If the new water is too warm or cold, it can also stress the fish, causing this behavior. Lastly, incorrect pH or chemical imbalances from the change can lead to surface swimming.

fish swim at top after water change

This page may contain affiliate links, which will earn us a commission. As an Amazon Associate we earn from qualifying purchases.

There are 7 main reasons why aquarium fish swim at the top of the tank after a water change such as chlorine poisoning, ammonia spike, chemical poisoning, temperature shock, low oxygen level, ph difference, or TDS shock.

Chlorine Poisoning

When you observe your fish swimming at the top after a water change, one possible cause could be chlorine poisoning. Chlorine is often present in tap water, which you might use to refill your aquarium.

Chlorine is added to our water to kill harmful organisms. It’s excellent for our drinking water, but not for fish. Our finned friends find it toxic; it can’t pass through their gills, blocking the normal oxygen flow.

How does it affect your fish?

  • Swimming erratically: They might dash about or even try to jump out.
  • Rapid gill movement: Fish use their gills to breathe. With chlorine poisoning, they’ll flutter more rapidly as they struggle for oxygen.
  • Swimming at the top: When fish fail to get enough oxygen, they swim at the surface, gulping air.

Long term, chlorine poisoning harms the fish’s gills, drastically reducing their lifespan. Sadly, even if the chlorine is removed or dissipates, the damage might already be done.

Therefore, always dechlorinate your water before introducing it to the tank. You can use a commercial product, or let tap water sit for 24-48 hours, allowing the chlorine to evaporate naturally.

Doing this simple step will keep your fish healthy, avoiding the dreadful consequences of chlorine poisoning.

Ammonia Spike

Let’s dive into the concept of an ammonia spike. This happens when there is a sudden increase in the amount of ammonia in your aquarium. Ammonia is a toxic byproduct of fish waste, decaying plants, and uneaten food.

During a water change, the balance between beneficial bacteria (the nitrogen converters) and the ammonia level could be disrupted. If there aren’t enough beneficial bacteria or they are removed, you could have a surge of unhealthy ammonia. In simpler terms, any disruption of your the nitrogen cycle can cause an ammonia spike.

The havoc created by this spike is a primary reason your fish swim at the top after a water change. High levels of ammonia irritate fish’s gills, causing them to gasp for air at the water’s surface. The signs include the fish gasping at the surface, showing lethargy, and loss of appetite.

Preventing an ammonia spike involves stringent tank hygiene:

  • Regular and moderate water changes, never replacing all the water at once.
  • Preventing overfeeding and promptly removing decaying organic matter.
  • Regularly testing water for ammonia levels.

Remember, an ideal ammonia reading is 0 parts per million (ppm), anything higher than that may harm your fish. A suitable home testing kit can assist you in avoiding any unexpected elevations in ammonia, keeping your underwater pals healthy and happy. Remember, a clear, balanced tank is the best environment for your fish.

Chemical Poisoning

When you conduct a water change, be sure about the purity of the water you’re using. Some tap water contains dissolved chemicals that could be harmful, making your fish swim at the top.

  • Heavy metals such as copper, lead, or zinc are common culprits. Most tap water contains traces of these elements. Fish are sensitive to these metals, which can lead to suffocation, hence their swimming to the surface.
  • Fluoride and Chloramine are another set. These chemicals commonly found in tap water also pose a risk.

To mitigate these issues:

  1. Use a water conditioner: This binds with heavy metals making them safe for fish, and neutralizing chlorine and chloramines.
  2. Check your water source: If you’re using tap water, be aware of what’s in it. In some regions, water providers must make this information available.
  3. Consider using filtered or bottled water: Filtering or purchasing clean water can provide a safer environment for your fish.

Recall that fish are delicate creatures, and any drastic changes can be lethal. Keep the water conditions stable and safe to allow your aquatic friends to thrive in your home aquarium.

Temperature Shock

During a water change, one primary reason fish may swim to the top of the tank is temperature shock. This occurs when the temperature difference between the old and new water is too great.

Fish are cold-blooded animals and they require a stable environment to thrive. Changes in temperature can lead to stress, and changes in metabolic rate, and even result in the death of the fish.

  • The new water should ideally be at the same temperature as the old one. To achieve this, fill a bucket with tap water and leave it in the same room with your aquarium to equalize the temperature before making the change.
  • Always use a thermometer to check the water temperature. You can’t rely on your sense of touch.
  • If the new water is too cold, you should adjust it very slowly, adding small amounts of the heated water gradually until it matches the existing water’s temperature.

Remember: an abrupt change of even 2-3°F (1-1.5°C) can cause temperature shock. By providing a stable environment with the right temperature, you’ll be ensuring the health and wellbeing of your fish after water changes.

Low Oxygen Level

A common cause compelling your fish to swim at the top after a water change is low oxygen level. The action of oxygen dissolving into water takes time, so immediately after a water change, the oxygen level will be low. Moreover, the surface is where the water absorbs oxygen from the atmosphere, drawing fish upwards in search of higher oxygen levels.

  • Fish breathe by extracting oxygen from water. So, when the water lacks sufficient oxygen, fish start to gasp for air at the surface.
  • Smaller tanks are more susceptible to low oxygen levels, primarily due to limited water volume. In small tanks, lack of oxygen happens more quickly after a water change.
  • Often, stirring or agitating the water surface can help. This process promotes the diffusion of oxygen into the water.
  • Another way to keep oxygen levels stable is by adding live plants which generate oxygen through photosynthesis.

Hence, since oxygen is vital for fish health, it’s crucial to ensure adequate oxygen levels, particularly after a water change. The liveliness of the fish usually serves as a good indicator. If they appear more active after introducing air into the tank, chances are the oxygen levels were indeed low.

Consequently, making it a routine to keep the oxygen levels stable after a water change can ensure your fish thrive and don’t resort to surface swimming due to oxygen deficiency.

Please be aware that various factors can cause low oxygen levels, such as overstocking, elevated water temperature, or poor aeration. Thus, monitoring these factors alongside regular water changes is crucial for maintaining a healthy aquatic environment for your fish.

pH Difference

Have you ever thought about the importance of pH balance in an aquarium? Well, fish thrive in specific pH levels, and any abrupt changes can stress them out.

When you top up your aquarium, make sure you’re aware of the pH level of the new water. If the pH is different from your tank’s water, this is going to cause a shock to your fish. Unfortunately, the effect is similar to tossing a human into a pool of ridiculously cold or boiling hot water: It’s not pleasant!

Fish can start swimming to the surface, looking for a ‘comfort zone’. It’s a concerning sight, showing your fish are stressed and uncomfortable. So, it’s crucial to measure the pH level of new water before adding it to the tank.

What can you do to avoid this shock? Here are three simple but critical steps:

  • Check the pH of your tank regularly, at least once a week. Aquarium kits usually include testing materials, so make use of them!
  • Never add water with a different pH. Let’s say your aquarium’s pH is 7.0. You should aim for new water to have a pH between 6.7 and 7.3. More significant fluctuations will be hazardous.
  • Use pH adjusters if necessary. Sometimes, tap water might have a very different pH. Don’t worry: there are plenty of safe, effective pH adjusters available to help you with that.

In short, a pH difference is not a minor inconvenience for your fish—it’s severe distress. By ensuring a consistent pH level, you’re protecting your fish and helping them live healthy, stress-free life.

TDS Shock

Suddenly seeing your fish swimming at the top after a water change? Total Dissolved Solids (TDS) could be the hidden culprit. TDS is the sum of organic and inorganic substances dissolved in the water.

With the water change, TDS concentrations may take an unexpected leap; a quick escalation your pet wasn’t ready for. This fluctuation disrupts fish’s fluid balance, forcing them towards less intensive TDS areas, often at the surface.

Think of it as a “TDS shock”.

But, how does TDS affect aquatic flora and fauna? The table below highlights a few impacts.

Impact of High TDS Examples
Water Taste Even humans can taste if TDS is heightened. Too high, and it becomes intolerable.
Hard Shell Formation Higher TDS can accelerate shell growth in crustaceans. Imagine it disrupting your calcium equilibrium!
Toxicity Some substances in TDS may be toxic in high amounts.

One way of avoiding TDS shock is by using a TDS meter. This handheld device measures TDS concentration in ppm (parts per million). For freshwater fish, aim for 50-250 ppm. Marine species thrive best between 1500-3500 ppm.

In conclusion, taming TDS is crucial for a healthy aquascape. High TDS evokes stress, pushing the fish to the surface — a clear sign of danger. Avoid the chaos. Regularly measure and control TDS in your aquarium. Regular attention to TDS can ensure a happy, healthy aquatic family.

What Amount of Water to Change in Your Aquarium?

Determining the correct amount of water to change in your aquarium is an essential task. First and foremost, the general consensus is to change 10-15% of tank water weekly, or 25-30% monthly. This routine minimizes the shock to your fish, thereby reducing the likelihood of them swimming at the top of the tank post-change.

An easy to remember guideline is:

  • Weekly: Change about 2 to 3 gallons (7.5 to 11 liters) of water for every 20 gallons (75 liters) of total tank volume.
  • Monthly: Change about 5 to 7 gallons (18.9 to 26.5 liters) of water for every 20 gallons (75 liters) of total tank volume.

Keep in mind, this is a guideline, not a strict rule. The exact amount can vary, it depends on:

  • Tank Size: Smaller tanks accumulate toxins faster than larger ones. Hence, more frequent water replacements might be necessary.
  • Fish Population: A heavily populated tank will require more frequent water changes than one that’s sparsely populated.
  • Fish Type: Some species are more sensitive to water changes than others.
  • Tank Condition: Tanks with problems, such as algae outbreaks or disease, may need more or less frequent water changes.

Doing a water change is of crucial importance in maintaining a healthy aquarium. But remember, be careful not to change too much water at any given time, as it might stress the fish and manipulate the stability of your tank environment. So, it’s better to do smaller, more frequent water changes than a large one abruptly.

A good balance and understanding of your aquarium’s needs will go a long way to ensure your fish live in a clean, healthy, and stress-free environment. Always monitor your fish after any water change to ensure they’re not feeling uncomfortable.

How do Fish Normally Act After a Water Change?

Usually, fish return to their normal behavior shortly after a water change. They continue their regular activities such as exploring, feeding, and interacting with other fish.

However, if the fish exhibit sudden changes in behavior post water change, this could be indicative of potential problems. Here are some manifestations to watch for:

  • Swimming towards the top: This generally indicates oxygen deprivation.
  • Gasping at the surface: Fish do this when deprived of oxygen or dealing with chlorine or ammonia poisoning.
  • Listless or lethargic movement: Often, this behavior is a sign of drastic temperature change or shock.

On the other hand, healthy behaviors include:

  • Displaying bright colors: Vibrant colors are a good sign of health in most species.
  • Maintaining an upright position: A fish should be able to maintain its balance in the water.
  • Showing a healthy appetite: Regular feeding patterns indicate the fish is comfortable in its environment.

Remember, observing your fish’s behavior post water change is crucial. In doing so, you can ensure their wellbeing and swiftly identify any issues.

How to Tell if Fish Are Stressed After Water Change?

Determining if your fish are stressed after a water change can be straightforward, but it often requires you to pay close attention. Here are some key signs:

  • Decreased or Increased Activity: Fish may become either lethargic or exceedingly active when stressed. If your fish is hovering or pacing more than usual, this could be an indicator.
  • Odd Swimming Behaviour: Fish swimming at the top or repeatedly jumping could be reacting to a recent water change. Sudden, erratic movements are often signs of stress.
  • Changes in Eating Habits: Stressed fish may stop eating or eat more than usual.
  • Marked Appearance Changes: Look for faded colors or visible wounds. These physical symptoms could denote stress.
  • Gasping at the Surface: If the fish is frequently at the top trying to get air, this shows a lack of oxygen in the water.
  • Evidence of Disease: Clamped fins, white spots, or a bloated appearance could indicate stress-induced diseases.

Above all, noticing any behavioral or physical changes that are out of the ordinary for your fish can point to stress. Remember, timely action can help to ensure your fish’s well-being after a water change. Awareness of these signs is crucial as it can lead to quicker resolution and a healthier aquatic environment.


Numerous factors such as chlorine poisoning, ammonia spikes, temperature shock, and pH differences can cause fish to swim at the top after a water change. It’s crucial to closely monitor your aquarium conditions, mindful of temperature, chemicals, and pH levels for the well-being of your fish.

Feel free to leave a comment – are there any other reasons you’ve noticed your fish behaving differently after a water change?

Leave a Comment

Your email address will not be published. Required fields are marked *