Why is there red in my fish? - The Environmental Literacy Council (2025)

Seeing red on your fish can be alarming for any aquarist. It’s a visual cue that something may be amiss, and it’s crucial to understand what those red hues could signify. The presence of red in your fish, whether it’s spots, patches, streaks, or discoloration, is often a sign of underlying health issues, ranging from minor irritations to severe infections. While sometimes a natural coloration, it’s important to recognize the differences and respond appropriately. Generally, red in fish can stem from:

• Inflammation: Redness is a classic sign of inflammation, often triggered by bacterial or fungal infections, parasites, or poor water quality.
• Hemorrhaging: This refers to bleeding under the skin or in the tissues, resulting in red spots or streaks. It’s commonly associated with severe infections or ammonia toxicity.
• Natural Coloration: Some fish species naturally possess red markings or hues, which are perfectly normal and a part of their genetic makeup. However, be sure to familiarize yourself with the natural colors of your specific fish to distinguish it from a symptom of illness.

Understanding which of these factors is at play will guide you toward effective treatment and preventive measures. The key lies in observation and prompt action when necessary.

Understanding the Causes of Redness

Bacterial Infections

One of the primary causes of red in fish is bacterial infections. These infections often manifest as red spots, sores, or ulcers on the fish’s body. These can start as small, red points on a single scale, and if left untreated can grow until they cause scales to fall off and expose the muscle tissue. A common culprit is a bacteria called Bacterium cyprinid, which is responsible for “red pest” disease. While this bacteria is usually present in most aquariums and ponds, it only becomes problematic when a fish’s immune system is weakened, usually due to stress or poor water quality. Symptoms of bacterial infections often include hemorrhaging (bloody patches) or open sores (ulcers), a white film on the fish’s body or fins, cloudy eyes, and tattered fins.

Ammonia Toxicity

Ammonia toxicity is another serious reason for redness in fish. This condition occurs when ammonia levels build up in the aquarium due to inadequate biological filtration, overfeeding, or overcrowding. Ammonia is a very caustic chemical, and can cause irritation and burning, especially around the gills, leading to inflammation and hemorrhaging around the gills which will present as a bright or dark red color. Affected fish may also exhibit red streaking on the body and fins, a cloudy appearance of the eyes, and will become listless and motionless often found on the bottom of the tank. This is a critical condition requiring immediate action to prevent fish loss.

Parasitic Infestations

Parasites, both external and internal, can cause redness and irritation. External parasites, like fish lice, can cause fish to rub against objects, leading to abrasions and redness. Additionally, they may cause cloudy eyes, white patches, or gasping for air, and generally make the fish become listless. Internal parasites can cause loss of appetite, listlessness, erratic swimming, and sometimes, you can see threadlike worms coming from the fish’s tail area. The irritated areas can easily be infected with bacteria, leading to secondary bacterial infections.

Fungal Infections

Fungal infections often present as gray or white patches on the skin or gills, which may later become brown or green as they trap sediment. While these are not typically red, they can contribute to secondary issues that do cause redness by further compromising the immune system and creating openings in the skin where bacteria can enter. Fungal spores are always present in our aquariums, but full-blown infections typically occur in fish that are stressed or injured.

Other Causes

Other potential causes of red include skin lesions from various etiologies, such as toxins, physical causes (abrasions or injuries), immunologic issues, or nutritional and metabolic problems. Sometimes the appearance of red is simply natural coloration, but it is important to observe the behavior of your fish and look out for other signs of illness, so that you are sure you aren’t misattributing the red coloration.

Action and Treatment

When you notice red on your fish, immediate action is crucial. Here’s a general approach:

1. Assess Water Quality: Poor water quality is a major contributor to fish health issues. Test your tank water for ammonia, nitrite, and nitrate levels. If any of these are out of balance, perform a partial water change immediately.
2. Identify the Symptoms: Closely observe the affected fish for other signs of illness like cloudy eyes, white spots, tattered fins, and abnormal behavior. This will help you narrow down the cause.
3. Treatment:
   • Bacterial Infections: Medications like API MELAFIX™ or API E.M. ERYTHROMYCIN™ are generally effective.
   • Ammonia Toxicity: Perform large partial water changes and consider using an ammonia remover product. Also, be sure that your tank is adequately filtered.
   • Parasites: Treat with appropriate antiparasitic medications or as recommended by your fish veterinarian.
   • Fungal Infections: Products like Maracyn, Ich-X, or API FUNGUS CURE are useful. Salt treatments can also be beneficial due to their broad-spectrum antimicrobial properties.
4. Salt Treatment: Sodium chloride (NaCl), commonly known as aquarium salt, is one of the best treatments against bacteria, fungus, and external parasites. Use non-iodized salt without additives. It can be used in various concentrations, from low to high, depending on the type of treatment needed.
5. Quarantine: When possible, isolate affected fish in a separate tank to prevent the spread of potential diseases and to allow targeted treatment.
6. Improve Tank Conditions: Maintain a stable environment, avoid overcrowding, and use a high-quality filtration system. Regularly test your water for abnormalities, and perform regular maintenance tasks like water changes and gravel vacuuming.

Frequently Asked Questions (FAQs)

1. Are red spots on fish safe to eat?

No, fish with red spots, especially if they appear as open sores or ulcers, should not be eaten. These are signs of disease and possible bacterial infection that could be harmful.

2. What is red pest disease in fish?

Red pest disease is a bacterial infection caused by Bacterium cyprinid. It is widespread, but only becomes harmful when a fish’s immune system is weak. It appears as red spots or sores on the fish’s body.

3. How do you treat red spot in fish?

Treatment usually involves improving water quality, performing water changes, removing infected fish, and adding salt to the water. If it is severe, medications specifically designed to treat bacterial infections can be used.

4. What causes red sores on fish?

Red sores can be caused by either a protozoan or a bacterium, or can be secondary to injuries from parasites or abrasions from their tank environment.

5. Why does my fish have red gills?

Red gills are often a sign of ammonia poisoning or a bacterial infection. Inflammation and hemorrhaging around the gills cause the redness.

6. What does a bacterial infection in fish look like?

Bacterial infections manifest in various ways, but common signs include a white film on the fish’s body or fins, cloudy eyes, tattered fins, and hemorrhaging or open sores.

7. Can salt cure fish fungus?

Yes, salt (sodium chloride) is an effective treatment against bacteria, fungus, and external parasites. It’s affordable, accessible, and can be used in varying concentrations.

8. How can you tell if a fish is infected?

Common signs of disease include disorientation, lack of appetite, white spots, discolored gills, difficulty breathing, bulging eyes, or mucus accumulation on the body.

9. How do I know if my fish has a bacterial or fungal infection?

Bacterial infections often show as body slime, mouth fungus, open lesions, swelling, fin and tail rot, and blood spots. Fungal infections appear as gray or white patches on the skin or gills.

10. Can fish recover from bacterial infection?

Yes, with proper treatment and improved water conditions, fish can recover from bacterial infections. Using Aquilibrium salt during treatment can assist in the healing process.

11. What does ammonia poisoning in fish look like?

Symptoms of ammonia poisoning include the fish becoming listless and motionless, red streaking on the body and fins, and cloudy eyes.

12. What are four signs of disease in a fish?

Four common signs include bruised-looking red hues, open sores, bulging eyes, and unusual behavior.

13. How do I know if my fish has parasites?

Signs of external parasites include cloudy eyes, white patches, gasping for air, rubbing on objects, and listlessness. Internal parasites may cause loss of appetite, erratic swimming, and threadlike worms coming from the tail area.

14. What does a sick fish look like?

Sick fish may have open sores, white patches, lumps, or spots on their body. They may also exhibit swollen or protruding eyes, raised scales, or an abnormally swollen or hollow belly.

15. What kills fungus on fish?

Maracyn, Ich-X, and salt are effective treatments against fish fungus. API FUNGUS CURE also works well, and in some cases, secondary bacterial infections.

By understanding the potential causes of redness in your fish and responding promptly, you can maintain a healthy and thriving aquarium. Regular monitoring, proper water quality maintenance, and timely intervention are vital for the well-being of your aquatic pets. Remember, observation is key to catching and addressing health problems before they become severe.