How to Prevent and Treat Common Aquarium Diseases
Apr 7th 2025
Setting up an aquarium is fun, but fish need consistent care to stay healthy. Sometimes, despite all our efforts to keep an aquarium clean and fish properly fed, our fish get sick. Aquarium fish diseases are more common than you may suspect. From your smallest tetra to your largest koi, caring for your fish begins with understanding how these diseases work and doing everything you can to prevent and treat common aquarium diseases expediently.
The Importance of Preventing Aquarium Fish Diseases
Fish illnesses are rarely an isolated occurrence. Nearly every illness stems from some imbalance or problem in the fish tank, causing stress to the inhabitants and making them susceptible to infection. In some cases, ill fish (and the types of parasites they can potentially host) can go unnoticed for a long period of time. Of course, making an effort to avert disease from happening is so much more efficient than treating an illness in the aftermath. And while treating these illnesses is not impossible, it can be costly and time-consuming. If proper action isn't taken at the first signs of illness, some treatments may not be effective.
Causes of Fish Diseases in Home Aquariums
Every illness a fish gets can be traced back to one reason. Root causes can include, but are not limited to, the following:
- Skipping quarantine: New fish, especially wild-caught varieties, may have hidden parasites or infections. Quarantining before introducing new fish to an established population is a must.
- Incomplete nitrogen cycle: Any amount of ammonia or nitrites can quickly stress and weaken a fish’s body. Ammonia and nitrite should be at zero before adding livestock to an aquarium. It is essential to wait until the nitrogen cycle is complete before introducing fish to an aquarium.
- Overfeeding and overstocking: Waste builds up in an aquarium due to too many fish or food breaking down in the water column, causing nitrates to spike quickly. An overstocked and/or overfed tank will have poor water quality, making your livestock susceptible to infections and stress-related illnesses.
- Neglecting water changes and maintenance: Aquariums need regular partial water changes to remove nitrates and fish waste. Filter media needs to be cleaned so the filter can function properly. Clear water does not equal clean water.
- Using shared tools: Your hand, nets, or even your siphons can introduce and spread certain diseases from one tank into another.
The Link Between Tank Conditions and Fish Illness
Fish are not meant to live in water systems where conditions dramatically fluctuate. When the temperature changes, pH fluctuates to extremes, or nitrogenous waste builds up, a fish's immune system will suffer. Insufficient water quality damages the fish from the inside. Even minuscule ammonia or nitrite concentrations make it more difficult for the fish's gills to work, and prolonged exposure will cause irreparable damage.
Fish gills allow the bloodstream to absorb oxygen from the water, and if damaged, the entire respiratory system will have to work harder to absorb and distribute oxygen. This ultimately causes a slower metabolism, increased stress, and reduced resistance against bacterial, fungal, and parasite infections.
Stress does not only originate from water quality; it can also come from:
- Aggressive, bullying tankmates
- Dramatic temperature fluctuations
- Improper photoperiods (day/night cycle non-existent)
- Too few places to hide
- Infrequent feeding
- Water flow too high or too low for inhabitants
Prolonged stress affects your fish’s daily activities, including eating, movement, and respiration, making them more susceptible to infections and illnesses. Fish kept in a healthy, well-maintained, low-stress environment are less likely to fall ill. Many common fish diseases can be treated in a timely manner with a decent recovery chance while also protecting the remaining fish population within the aquarium.
The white specks on these Ember Tetras indicate that these fish have ich or white spot disease.
Common Freshwater Aquarium Fish Diseases Every Aquarist Should Know
Even in a well-maintained tank, your freshwater fish can quickly run into serious health issues. Whether running a peaceful community tank or raising a single betta, some illnesses creep in quietly and do damage if you don’t catch them early.
- Velvet Disease: This is caused by a parasitic infection, which causes a gold or rust film on a fish's body. This often leads to the fish physically rubbing against the tank's surfaces and showing telltale signs of lethargy. It has the propensity to spread quickly, requiring immediate treatment.
- Dropsy: This is an internal bacterial infection that causes rapid fluid accumulation. The fish appears blown up, with the scales protruding like pine cones.
- Swim Bladder Disorder: Swim bladder issues are generally noticed when fish float sideways, sink slowly, or swim spastically. It can be a result of overfeeding, infestation, or constipation.
- Gill Flukes: These microscopic parasites attach themselves to the fish's gills and make it hard for them to breathe. In response, fish battle with fast gill movements or gasp at the water's surface.
- Ich (White Spot Disease): One of the most common aquarium fish ailments, which is clearly observed by its features: white spots resembling salt in color laden on the body and fins of fish. It’s very contagious… but it can be easily treated.
- Fin Rot: The most common bacterial infection, which leads to the rotting of the fins. Generally, it results from inadequate water quality conditions and is common among injured or stressed fish.
- Columnaris: Often referred to as mouth fungus, this condition is actually caused by the bacterium Chondrococcus columnaris. It typically indicates poor water quality in the aquarium, making regular water changes essential if symptoms appear. Newly introduced fish are particularly vulnerable to this bacterial infection.
- Anchor Worm: Crustaceans that burrow into the fish's skin, causing irritation and lesions.
- Fungal Infection: Saprolegnia and other fungi often create opportunistic infections in stressed or injured fish. Common symptoms are cotton-like or fluffy growths on the fish's body or fins. The underlying cause is usually stress caused by poor water quality.
- Bacterial Infection: This can be identified by the appearance of various symptoms, including ulcers, red or bloody spots, and lethargy. The cause can be stress, poor water quality, or the introduction of a pathogen from an outside source.
Recognizing Symptoms of Common Aquarium Fish Diseases
Many fish diseases have signs that worsen with time and can escalate into significant issues very quickly. Identifying methods early in the procedure is extremely important to ensure the fish are saved.
Some of the most common warning signs of a disease include:
- Skin is covered in white fuzz, specks, and/or gold dusting.
- Receding and/or jagged fin edges.
- Red streaks or open sores are found on the fish’s body.
- Swollen stomachs and torsos.
- Fin clamping (the fins are held tightly to the body).
- Tiredness or being passive, floating onto the water's surface, or resting on the bottom.
- Scratching against rocks or tank decor.
- A change in overall appetite.
Keeping Your Aquarium Disease Free
Creating a routine is the best form of prevention. Below are a few tips for maintaining the health of your aquarium fish.
- Quarantine new fish for 3-4 weeks before their addition to the main tank.
- If keeping multiple aquariums, do not share nets or equipment without disinfecting.
- Keep fish appropriate for your aquarium size, and do not overstock.
- Research compatibility before keeping different species of fish together. Also, consider whether a species prefers to be in schools, in pairs, or as the only one of its species.
- Feed the fish appropriately; neither too much nor too little.
- Dechlorinate any tap water you add to your aquarium.
- Ensure the temperature is stable and appropriate for the species being housed.
- Consider using beneficial bacteria and live plants to enhance water quality.
Water Quality: The Best Offense is a Good Defense
Filters help remove debris from the water column and enable good bacteria to flourish, making it easier for nitrification. Nitrification is when ammonia (the main byproduct of fish waste and respiration) is converted to less harmful nitrates. Filters also move the water around, ensuring there are no areas of the aquarium where water becomes stagnant and oxygenation increases. Clean, stable water is the most important aspect of preventing fish diseases from flourishing.
At a minimum, you should monitor:
- Ammonia (should be 0 ppm).
- Nitrite (0 ppm).
- Nitrate (ideally under 40 ppm).
- pH (species-specific).
- Temperature (stable and species-appropriate)
- kH (depletion will kill beneficial bacteria and disrupt stability).
If you're dealing with an illness in your tank, poor-quality water is likely the culprit. Medications will not be effective if the previously mentioned parameters are not stabilized and corrected. If you don’t know your water parameters, a water test kit can be purchased at most fish stores (including Fitz’s Fish Ponds). Stress doesn’t look the same in fish as in humans, but it's just as serious. Fishes forced to live in poor water conditions become stressed and more susceptible to illness.
You can reduce stress and improve water parameters with:
- Proper lighting cycles (no more than 8 hours; set lights on a timer).
- Ensure stocking levels and tank mates are appropriate.
- Do not overfeed your fish.
- Conduct small, frequent water changes (about 10 - 20% weekly).
- Clean your filter media regularly (at least once per month).
- Ensure your heater is functioning properly, and get a thermometer to monitor the temperature.
- Use a dechlorinator when adding new tap water.
- Use a beneficial bacteria culture to improve your biological filtration rate.
If your fish is in a healthy aquarium with optimal water parameters, it can recover quickly from mild illnesses with the help of over-the-counter fish medications. It cannot be stressed enough that a disease cannot be treated until the water parameters are corrected.
Role of Aquarium Plants and Beneficial Bacteria in Preventing Fish Diseases
Along with making your tank beautiful, plants do a lot more. They provide a more natural place for fish to live (reducing stress), compete with algae, and absorb nitrates. Beneficial bacteria, particularly if they live in your filter media and aquarium substrate, work to transform harmful ammonia into less toxic nitrates. Ammonia can increase to dangerous levels in their absence with even minimal feeding.
So, remember to rinse filter media only with tank water, cycle your tank properly before introducing fish, and perform regular water changes. Adding plants and beneficial bacteria cultures like Microbe-Lift Special Blend will help boost your aquarium’s efficiency and keep it cleaner between maintenance sessions.
Floating plants are low maintenance and great for absorbing excess nitrates.
Strategies for Long-Term Care of an Aquarium
A thriving aquarium doesn’t run on autopilot. Behind every crystal-clear tank and happy school of fish is a set of tried-and-true habits that keep everything balanced over time. Long-term care isn’t complicated, but it takes consistency and planning, from keeping water parameters steady to a working cleaning schedule.
Here’s what it takes to keep your tank healthy for the long haul:
- Keeping track of every fish’s health daily. Observe them when they eat to see if anything is “off” or irregular.
- Perform maintenance and water changes regularly. Small, frequent water changes work wonders.
- Provide food that meets the necessary standards of the species residing within the tank.
- Remove any uneaten food so it doesn’t spoil the water.
- Test your water parameters whenever there is anything suspicious.
- Quarantine all new livestock for 3-4 weeks and keep a quarantine aquarium.
Setting up a Quarantine Tank
If you're new to the freshwater aquarium hobby, one of the most important steps in keeping your fish healthy is setting up a quarantine tank. A quarantine tank is not something that needs to run full-time like your display tank—instead, it’s a temporary setup used when you bring home new fish or need to treat sick fish. This helps prevent the spread of disease to the rest of your aquarium. A standard 10-gallon fish tank offers enough space for small to medium tropical fish while allowing easy and accurate medication dosing.
Essential quarantine tank equipment includes:
- 10-gallon aquarium with a secure lid (reduces evaporation and prevents jumping fish).
- Aquarium heater (25–50 watts is ideal) as well as a reliable thermometer.
- Sponge filter (cycled from your main tank).
- Air pump and airline tubing.
- Dedicated aquarium nets and scrubbers (used only for the quarantine tank).
- Aquarium salt (for treating stress and minor issues).
- Common fish medications (for treating bacterial, fungal, and parasitic infections).
Tips for a successful quarantine setup:
- Cycle your sponge filter in your main tank at least a week in advance to establish beneficial bacteria; which are vital for stable water parameters.
- Add a dose of beneficial bacteria when setting up to jump-start the nitrogen cycle.
- Ensure your water temperature matches the requirements of your new fish before introducing them.
- Always read medication labels carefully and avoid mixing treatments unless given professional advice. Treat one condition at a time to avoid harming your fish.
A quarantine tank is crucial to good fishkeeping practices; whether you're keeping tetras, guppies, bettas, or other community fish. It gives you a controlled environment to observe and treat new arrivals before introducing them to your planted aquarium. By taking this extra step, you help ensure a healthier and more stable ecosystem for your aquatic pets.
Some Treatment Options
Whether you're just starting with a freshwater-planted aquarium or you are an experienced aquarist, having the right fish medications is essential for maintaining a healthy tank. At Fitz’s Fish Ponds, we stock four trusted, broad-spectrum treatments that are effective for most common fish illnesses. These products are safe for both freshwater tropical fish and aquarium invertebrates and—aside from aquarium salt—are also plant-safe, making them ideal for use in planted aquariums.
While we are not veterinarians and cannot diagnose your fish or recommend specific treatment plans, these over-the-counter fish medications come with detailed instructions for safe and effective use. Always follow dosage directions carefully, and if your fish does not improve or symptoms worsen, we strongly recommend consulting a qualified aquatic veterinarian for proper diagnosis and care.
Here are the four recommended fish treatments we always keep in stock:
- Aquarium Salt: A staple in any aquarist’s toolkit, aquarium salt helps treat mild fish illnesses, boosts slime coat production (your fish's natural defense), supports gill function, and enhances the absorption of other medications. It’s safe for most freshwater species but should be used cautiously in tanks with delicate plants or sensitive invertebrates.
- API E.M. Erythromycin: This broad-spectrum antibiotic is plant-safe and effective against a variety of bacterial infections, including mouth fungus, body slime, Furunculosis (open sores and swelling), bacterial gill disease, and hemorrhagic septicemia (red streaks or blood spots). It can be used alongside Ich or fungus medications to help prevent secondary bacterial infections.
- API General Cure: An essential treatment for internal and external parasites in freshwater fish, this medication targets gill and skin flukes (Dactylogyrus and Gyrodactylus), velvet disease, and the infamous hole-in-the-head disease (caused by Hexamita and Spironucleus). It is safe for use in planted aquariums and with most tropical species.
- ICH-X: A powerful yet gentle remedy for treating Ich (white spot disease) and other protozoan infections, ICH-X uses a safer form of Malachite Green, making it suitable for scaleless fish and sensitive species. It also helps control fungal problems in stressed or injured fish.
These freshwater aquarium medications are a great starting point for hobbyists dealing with common fish health issues. Keeping your aquarium stocked with these essentials ensures you're ready to act quickly at the first sign of trouble. For persistent or unusual symptoms, don’t hesitate to seek advice from an aquatic veterinarian—early intervention can make all the difference in saving your fish and protecting your entire tank.
Frequently Asked Questions (FAQ)
What are the most common- diseases that affect fish in freshwater aquariums?
Some common diseases of fish kept in aquariums include ich, velvet, columnaris, dropsy, fin rot, and swim bladder disease.
How do I know if my fish is ill or just under stress?
Look for external signs of illness, such as spots, bloating, and abnormal swimming behavior/movement. Stress is usually associated with hiding, refraining from eating,
Should I isolate newly purchased fish before introducing them to my tank?
Absolutely. Three to four weeks allows you to monitor and treat any possible diseases without risk of infecting your main tank.
Can I treat all fish diseases with aquarium salt?
It does not work for all diseases, but it works well for simple cuts, stress, and some bacterial or parasitic infections. Always follow the directions given.
What are the best prevention measures against diseases common in aquariums?
Keep up with your aquarium maintenance, including partial water changes every two weeks. Also, ensure your temperature is stable, and perform a water test if something seems off.
What materials will I need for the construction of a quarantine tank?
A 10-gallon aquarium with a lid, beneficial bacteria, sponge filter, air pump, airline tubing, heater, and thermometer is required. Substrate and decor are not required.
What steps should I take if a single fish becomes sick and the other fish does not?
Try to separate the sick fish into a quarantine tank. At the same time, the water quality in the main system should be tested, and anything causing the illness should be addressed. Treat the fish in quarantine with medication, and continue to monitor the health of fish in the main tank.
Need Help Keeping Your Fish Healthy? Let Fitz Fish Ponds Handle It!
Caring for a backyard koi pond or a freshwater aquarium is a relaxing pastime, but like every hobby it comes with its hurdles, such as diseases. The good news is that Fitz’s Fish Ponds provides aquarium services, and our Aquariums & Aquascaping stores have everything you need to build and maintain a successful planted aquarium.
If your tank isn’t thriving, then it isn’t complete! Contact the experts at Fitz Fish Ponds today to build the aquascape of your dreams! You can also call us at (908) 420-9908.
