Angelfish Swimming on Side

If your freshwater angelfish is swimming sideways, common problems can include things like:

  • Swim bladder disease
  • Damaged fins
  • Bacterial infections
  • Bloating
  • Parasites
  • Genetics/Deformities

Freshwater angelfish (Pterophyllum species) make fantastic pets with their unique body shape, colorful patterns, and graceful movement. While the species is known to be hardy, it can develop problems like any other pet. One issue that can send you into panic mode is finding your angelfish swimming on its side.

Through years of fish keeping we have found more than one fish on its side. We empathize with you and your pet and would like to share some ideas about what might be causing the problem…

Why Is My Angelfish Swimming Sideways

It has damage to one or more fins

Your pet uses its fins to stay upright in the water column and to move in three dimensions through it. It is understandable then that when one or more of these are damaged, it could explain your angelfish swimming on its side.

There are several of these appendages on your angelfish:

  • The anal fin: This large, single structure sits on the bottom of the fish, adding to its overall height. It stabilizes the fish while it swims.
  • The caudal fin: The caudal is the fan-shaped tail fin. This singular appendage propels the fish through the aquarium.
  • The dorsal fin: This is the large fin atop your angelfish that helps give its height and shape. Your pet uses it to maintain balance when it swims.
  • The pectoral fins: These are a pair of fins found on the sides of the fish. They decrease and increase speed side-to-side, alter direction, and slow the fish down while it swims.
  • The pelvic fin: These fins are the pair of long, stringy appendages in front of the anal fin along the bottom of the fish. Freshwater angelfish use these to move up or down in the water column and for stabilization.

Damaged fins are an ideal place to start your inspection because they are visible. You can look for fins that are not moving or appear to be overworking. There can also be visible damage on the spins or rays of the fin.

Things in the tank that can cause damage include fin nipping by tank mates (including other angelfish) or decor and substrate the fins make contact with. The damaged fin is now susceptible to infection, leading to fin or tail rot.

We recommend doing bi-weekly water changes like you would for most fish illnesses. Some keepers prevent infections by adding aquarium salt, and there are products like API Stress Coat that promote a healthy protective coat.

Many angelfish swimming

Bacterial infections

Bacteria can cause symptoms that make your fish swim on its side. Several contribute directly, while others can create a chain reaction that results in your angelfish swimming on its side. Some of the common bacterial infections we have experienced while keeping tropical fish include:

  • Aeromonas: These bacteria infect one or more organs in the fish, along with ulcers on its scales. Water quality and nutrition play a role in its onset, and symptoms can include abnormal swimming.
  • Vibriosis: We see the bacteria causing vibriosis more in wild-caught specimens than captive-bred. It can cause bleeding and ulcers on the fins and scales of your angelfish. The infection can lead to bodily changes, including the liquefaction of internal organs.
  • Edwardsiellosis: This bacterial infection infects the skin and intestines in amphibians, fish, mammals (including humans), and reptiles. Gas-filled lesions make it difficult for angelfish to swim normally.

Your angelfish can become infected by bacteria due to stress. Factors can include water parameters like water temperature extremes and poor water quality. Other things that promote bacterial infections include parasites and poor nutrition. Tanks mates can also infect your freshwater angelfish as the bacteria enters the water column through feces, open wounds, and dead fish.

Bloated stomach

Bloating is another potential reason you see an angelfish swimming on its side. It is a state where your pet’s stomach or digestive tract swells, in this case, to the point it negatively impacts its swimming.

A sick fish suffering from infections or parasites might bloat. The bloating will only subside by treating the underlying condition. That will improve the fish’s health and return it to a natural swimming state.

Constipation is another cause of bloating in freshwater angelfish. The commonly held belief behind this is a lack of fiber in the diet. High-fiber foods (including peas from a tin) can be used as a natural laxative to help your pets pass feces.

When inexperienced angelfish keepers try to provide their pets with a proper diet, they sometimes overfeed.

It does happen, especially with flake and freeze-dried foods that expand in the water and stomach. You can pre-soak foods to help negate expansion and feed only enough fish can consume within about two minutes. If you suspect bloating from overeating, don’t feed your fish for 24 to 48 hours and see if that helps.

Angelfish feed at the top and mid-levels of your aquarium. They might inhale air when grabbing floating foods at the surface. Pre-soak foods or switch to something that does not stay at the surface.


Unfortunately, parasite infestations are common in the fish-keeping hobby. We have dealt with a fair share of these during our years of keeping tropical fish, and there is no doubt parasites can cause issues that could explain why your angelfish is swimming weird.

While most parasites you may come across are external, there are a few internal parasites around. Three of the more common ones we have seen over the years in freshwater fish tanks include:

  • Chilodonella: This parasite attacks the gills and can be deadly when not caught in time. One side effect is that fish with Chilodonella scratch themselves against objects, which could damage fins and offer pathways to fish rot later.
  • Flatworms: Two common forms of flatworms are Dactylogyrus and Gyrodactylus. They create ulcers on the gills or scales of your angelfish. You will have to get medicated water from a vet to clear them.
  • White spot disease: Also known as Ich, these parasites attach to the body, gills, and fins of fish. They can be a problem because they weaken angelfish and cause them to rub their fins against objects, causing further issues.

Parasites enter your tank through new fish, plants, live foods, or other items.

We recommend washing anything before putting it into your aquarium. You should also quarantine new tank mates before putting them into an established community tank.

Live foods can be a valuable nutrient source for tropical fish. Many owners consider them essential for a proper diet, but they can carry bacteria and parasites that infect your pet’s digestive tract or enter the water column through feces. Dried and frozen foods are great alternatives that still offer those essential nutrients without the unwanted guests that create sick fish.

Genetics and internal deformities

While genetic issues usually appear in an angelfish at the juvenile stage or sooner, abnormalities can manifest during your pet’s lifecycle at any stage. These can range from developing weakness due to poor nutrient absorption to abnormal organ or tissue growth. The underlying condition has likely always been there, but it worsens over time and causes neutral buoyancy issues that put your fish on its side.

Problems can also develop through mutations caused by internal changes affecting neutral buoyancy. Cancers and tumors might develop within the fish, making swimming a challenge. These can be due to genetics, but they can also develop due to external factors like viral infections.

These conditions can be more challenging to determine when compared to the other issues we discuss here. External growths are easy to see, but many tumors (benign in one area or malignant that spread) are only visible if they create bumps on the scales above.

Treatments for most of these will require the services of a veterinarian. Surgeries can remove some external or internal growths. Other traditional cancer therapies carry a price tag beyond most keepers’ budgets.

Can you prevent any of these issues from developing? Keeping the aquarium clean with frequent water changes and maintaining proper water parameters can help, especially with viral infections. You will not be able to prevent genetic issues from manifesting, however.

Swim bladder disease

Swim bladder disease (or swim bladder disorder) is an umbrella term to describe anything that impacts the swim bladder, causing your angelfish to swim upside down or on its side. Causes include fluid retention in the organ, inflammation, or other bodily factors preventing the bladder from functioning correctly.

The bladder gets used by the fish to maintain its buoyancy. It fills with gas (air) to create positive buoyancy to move up the water column or deflates to create negative buoyancy to move toward the bottom.

If you notice the fish is moving its fins but having problems staying upright, that can be a clue it has swim bladder disorder. The key to treatment will depend on what might be causing the issue (bloating, infections, fluids in the bladder, stress).

Angelfish and Aquarium Health

As a freshwater angelfish enthusiast, you will spend lots of time watching your fish. Dedicate some of that enjoyment to observing its movement, skin, eyes, gills, and energy levels. If anything deviates, you know to take some extra time to verify you do not have a sick fish (and treat it if you do).

Proper water parameters go a long way toward keeping your angelfish happy and healthy. Provide plenty of room and keep the water temperature between 75 and 84 degrees Fahrenheit. Good water quality for this Amazonian species means a ph between 6.5 and 7.5, while the water hardness should stay between 5 and 12 dH.

Quarantine new fish to prevent introducing problems into your community tank. You can also substitute frozen foods for live to avoid unwanted bacteria or parasites from entering the water. We also recommend up to 25 percent water changes at least once a week.


If your angelfish is swimming sideways, it needs extra care to get it healthy. Damaged fins, swim bladder issues, infections, and parasites can make it hard for your pet to swim. Identifying the problem and treating it is crucial to saving it.

Keep your tank clean, maintain proper water conditions, and avoid introducing things into the water without quarantine. Learn how your fish behaves and pay attention to its physical appearance. Changes in either can be the first sign of trouble.

We all experience oddly swimming angelfish in our aquariums; hopefully, you now understand why and what to do when it happens with your favorite pets. If you can any questions, ask away so we can get you the answers you need!


Why is my angelfish floating on its side but not dead?

Something is affecting its buoyancy or preventing it from mechanically swimming upright. Damaged fins or problems with the swim bladder are the usual culprits here. Your freshwater angelfish could be having issues with water parameters, overfeeding, infections, or internal organ damage.

Why is my angelfish lying on its side gasping?

Gasping indicates a lack of oxygen in the tank while floating is a sign of swim bladder issues. Inspect your fish for problems with fins, open wounds, or other signs of trauma. We would suspect there could be problems with the water, so check things like temperature and water quality with a test kit.

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