Freshwater Aquarium Disease Prevention, Recognition, and Treatment

Hobbyists who take proper care of their aquariums will rarely encounter sick fish but there is no guarantee that you will never have to treat your fish for 540. disease. Almost all health problems can be avoided by maintaining proper water chemistry and pristine water quality, feeding a varied diet of high-quality fish foods, and providing fish with an optimum habitat and suitable tank mates. Knowing how to prevent diseases as well as being able to recognize and treat problems before they become unmanageable will ensure years of success and enjoyment with your aquarium. Keep reading to learn how to keep your fish healthy, spot problems early, and treat common fish diseases.


Virtually all fish diseases can be traced to stress, which weakens their immune systems.  Sources of stress include shipping, poor water quality, improper water chemistry, inadequate filtration, improper diet, overcrowding, leaving the light on 24/7, injuries, aggression from other fish, and inadequate habitat. To keep your fish in optimum health, perform regular partial water changes, be diligent about filter maintenance, feed them a varied high-quality diet, don’t overstock your aquarium, and put the light on a timer to simulate a normal day/night cycle. When performing water changes, always treat tap water with a conditioner before adding it to your aquarium.

Never purchase fish that have just arrived at your local aquarium store. New arrivals are usually stressed and moving them again will only stress them further. Let them settle in for a week or more before purchasing. Take new purchases straight home and acclimate them to your aquarium’s pH and temperature for at least 30 minutes, spending extra time with sensitive species or if the store’s water chemistry is significantly different from yours. Make sure there is plenty of cover for all your fish and rearrange decorations if necessary to deter established territorial fish from harassing new arrivals. After adding new fish to your aquarium, leave the light off for a few hours to allow them to adjust to their new environment. Do not tap on the glass or suddenly turn on the aquarium light in a dark room.

While most freshwater aquarium fish are raised in captivity today, some rarities and oddball fish are still collected in the wild. These fish may have a higher chance of carrying diseases and have typically gone through far more stress on their way to your aquarium. Extra care and quarantine measures should be taken when purchasing wild-caught fish.  When keeping burrowing fish such as eels in the genus Mastacembelus and certain types of loaches, use sand instead of gravel to avoid injury to their bodies when they burrow into the substrate.  Use smooth, rounded gravel to prevent Corydoras catfish, loaches, goldfish, and other species that like to forage on the bottom from injuring their barbels and mouths.


An effective way of maintaining a disease-free aquarium is to quarantine all new additions. While this may not be practical for every aquarist, it’s well worth the investment for those who keep high-value fish such as discus, rare fish or dedicated planted aquariums where the use of medications is not recommended. Quarantining new fish greatly reduces the chance of introducing a disease organism into your aquarium and allows you to safely treat sick fish if necessary, without introducing chemicals to your show tank. A quarantine aquarium can also be used to isolate bullies or fish that are being picked on.

A 20-gallon aquarium works for most situations. It should be filtered, heated, and maintained just like any other aquarium, and should be fully cycled with test fish before being used for new purchases or treatment. To provide cover for fish, decorate the aquarium with plastic plants or other non-porous decorations that are easy to sterilize and/or clean. Do not use porous rocks or driftwood as they can absorb medications. Different sized PVC fittings or sections of pipe can also be used as hiding places. Do not use gravel or substrate, as this gives parasites like Ich a place to reproduce.

Remove carbon and other chemical media from the filter, as they will adsorb medications, lowering their effectiveness. An aerator is recommended in addition to the filter, as some medications lower the oxygen level in the water. A light can be used but is not necessary, as dimmer surroundings will calm your fish and are also known to inhibit some disease-causing organisms.

Whenever you purchase new fish, place them in the quarantine aquarium for a minimum of 30 days to make sure they aren’t sick. Many hobbyists treat new purchases preventatively against parasites whether they are observed on their fish or not. Perform a 25% water change and filter with carbon for at least 48 hours before switching medications or introducing new fish to your quarantine station.

Have a separate net, siphon hose, an algae scraper, and other equipment for your quarantine aquarium, and never use them in your display aquarium. Doing so risks spreading diseases and defeats the purpose of the quarantine aquarium. Disinfect this equipment in bleach water and rinse well on a regular basis. After working in your quarantine aquarium, scrub your hands and lower arms with an anti-bacterial soap before working in your display tank.

Ultra-violet Sterilizers

Disease-causing organisms exist in virtually every aquarium, but they will not infect fish if their numbers remain low and the fishes’ immune systems are functioning properly. Ultra-violet sterilizers kill disease-causing organisms as well as suspended algae and help keep water healthy and crystal clear. Coralife Turbo-Twist UV Sterilizers are available in three sizes and accommodate aquariums up to 500 gallons. UV sterilizer lamps should be changed every 10 months or 7,000 hours of operation to maintain peak performance.


In order to recognize problems that may arise, it’s helpful to have an understanding of what “normal” appearance and behavior are for your fish. Observe your fish regularly – feeding time is a good opportunity to do this.  Look for white spots, cloudy eyes, bloody patches, a white body film, or torn, ragged fins. Also, there are certain things fish never (or at least very rarely) do.  For example, fish do not normally gasp at the surface. They typically do this because of poor oxygenation, high nitrate levels, parasites, or damage to their gills. Other behaviors that are cause for concern include loss of color, shimmying, rubbing against decorations or the substrate, cowering, refusing food, or dashing around the aquarium.

If your fish appear sick or exhibit abnormal behavior, consider speaking to an expert as soon as possible for assistance with diagnosis and treatment. Photographs or a short video of your fish can be extremely helpful when seeking advice, as verbal descriptions can be misinterpreted by aquarium shop personnel and other experts.


You can’t predict if or when your fish will get sick or what disease they might come down with, but it’s a good idea to be familiar with common ailments and their symptoms so you can quickly begin treatment if you think they may be infected. Being able to recognize problems and begin treatment early gives you the best chance of saving your fish!


“Ich” (Pronounced ICK) is short for Ichthyopthirius multifiliis, the most common aquarium parasite. The most noticeable symptom of Ich is small raised white spots on the body and fins, however rapid breathing, rubbing against the decorations or gravel, twitching, or darting around the aquarium can be early indicators of an infection. Common triggers for the onset of Ich are a sudden drop in temperature caused by heater malfunction or adding cold water during a water change or introducing new fish. Incoming fish can be carriers of Ich without showing signs, but your aquarium can also have a latent population of parasites that existing fish have become resistant to. New fish additions are typically stressed and have lowered immune systems, making them more likely to become infected. Scaleless fish such as loaches, catfish, and eels, as well as silver-scaled fish such as silver dollars, hatchet fish, and Bala sharks, tend to be more susceptible to ich, but all fish can be affected. If you think one of your fish might be infected, you will need to treat the entire aquarium, as Ich is highly contagious. The earlier you recognize Ich and begin treatment, the better your chances are of curing your fish.

There are several ways to treat Ich. If you’re relatively new to fishkeeping, the most effective and safest way is to use an aquarium-safe Ich medication. First, make sure the temperature of your aquarium is appropriate for the types of fish you keep (76° to 80° F for most tropical fish).  Remove carbon and other chemical media from your filter and follow the recommended dosing instructions for the net gallonage of your aquarium. Remember that after adding gravel and decorations, your aquarium holds 10% to 15% less actual water than its stated size! If repeated treatment is called for, perform a 20% water change and vacuum the gravel to remove parasites before each subsequent dose. Make sure replacement water is the same temperature as the water in your aquarium.

Once you’re sure your fish are Ich-free, do another 20% water change and return carbon and other chemical media to your filter. Observe your fish carefully for the next few days to make sure they don’t experience a relapse. If white spots persist after 5 days of treatment, it’s possible that a secondary bacterial infection has invaded the lesions left by the parasite, and an antibiotic is called for. (See bacteria treatment below)

Another method practiced by experienced hobbyists and those who don’t like to use chemicals is to gradually raise the temperature in the aquarium to approximately 85° F and maintain it for up to two weeks. The heat speeds parasites through their life cycle, and they eventually die off.  Adding an air diffuser to the aquarium is recommended when using this method, as water holds less dissolved oxygen as the temperature rises. NOTE: Only increase the temperature by 1 to 2 degrees per day and watch your fish closely for signs of distress. If they begin to gasp at the surface or show rapid or labored breathing, slowly lower the temperature until they recover and consider an alternate treatment method. Also, recent reports suggest that heat-resistant strains of Ich are becoming more common in the hobby, making this treatment method less effective.

Ich can also be treated with aquarium salt or unionized salt. Some species of freshwater fish are sensitive to salt, so it’s important to do research before deciding on this treatment method. In addition to killing parasites, salt effectively lowers stress by reducing ammonia and nitrite toxicity in freshwater aquariums and can help bolster freshwater fishes’ immune systems. For general Ich treatment, a ratio of 1 tablespoon per 5 gallons should be used. To avoid stressing fish, dissolve small amounts of salt in water and add the solution to your aquarium gradually over a 24-hour period until the full dose has been administered. Observe fish closely and discontinue if they show signs of stress. When doing water changes, add 1 tablespoon of salt per 5 gallons of water changed.

Treat only the replacement water, not the whole aquarium.

Raising the temperature in conjunction with the addition of salt is also practiced but remember that both of these measures lower the dissolved oxygen level in the water. Regardless of which treatment method you choose, be sure to vacuum the gravel every few days to remove incubating parasites.


This protozoan parasite produces similar symptoms to ick, such as labored breathing, gasping at the surface, clamped fins, heavy mucous secretion, and scratching on objects in the aquarium.  An outbreak of Chilodonella is often associated with poor water quality. Discus and young fish tend to be more susceptible, but they can infect any fish. Effective treatments include formaldehyde, methylene blue, and acriflavine. Raising the temperature does not typically provide any benefit.


Also known as Velvet, Rust, or Gold Dust disease, there are several species of this dinoflagellate that attack fishes’ skin and gills. Symptoms include what appears to be a yellow to rust-colored dusting on the fish’s body, heavy mucous secretion, clamped fins, and/or labored breathing or gasping for air. Unfortunately, by the time symptoms are observed, the chances of saving the fish are limited and massive die-offs from this disease are not uncommon. The best treatment for Oodinium is Acriflavine. It should be noted that this product stains the water in an electric yellowish-green that persists for a long time. Water changes and the use of carbon in your filter will eventually clear the water. Copper sulfate can also be effective, but remember that it is lethal to shrimp, snails, and most live plants, and is not as effective in soft water. Raising the temperature to 86° F is also known to kill Oodinium, but this lowers the oxygen level in the water, which is not recommended with already heavily stressed fish.

Fish Lice

Fish lice are crustaceans in the genus Argulus. They have a broad flat shell, four sets of swimming legs, and are easy to see. Infected fish may swim erratically or rub up against objects in the aquarium in an attempt to remove them. Upon close inspection, you may be able to see the lice moving around on your fish! Fish lice are usually brought into the aquarium on pond-raised or wild-caught fish. They occur most often on goldfish and koi, but they can infect any freshwater fish. Argulus attach to the fish’s body and begin digesting its body tissue. Severe infections are particularly damaging and can lead to secondary bacterial infections. Fish lice can be physically removed using tweezers, however, the aquarium should also be medicated to kill any eggs that have been laid. Dimilin is known to be effective against fish lice.

Anchor worm

Lerner Coprinaceae, known as Anchorworm, is another crustacean commonly seen on goldfish and koi, but they can infect any fish. They have multiple life stages, but infestation begins as what looks like a pimple or red sore on the fish, then later the string-like bodies of females can be seen attached to the fish. Anchor worms bore into the fish’s body through the scales, attach using hook-like appendages and begin to digest body fluids. Anchor worms also invade the fish’s gills and mouth cavities. They can be removed using tweezers, but care must be taken to pull the entire parasite out, as they sometimes break off, leaving the head and “anchor” attached. When pulling Lerner out of a fish, a small chunk of flesh is sometimes removed, requiring the use of antibiotics to prevent secondary bacterial infections. To prevent further infestations, the aquarium should be treated with salt or Dimilin to kill eggs and other life stages.


Fish flukes (Gyrodactylus sp.) are a group of parasites that invade the skin and gills of aquarium fish. They attach using a set of “hooks” and begin sucking blood and body fluids, thus weakening the fish. Symptoms include loss of color, difficulty breathing, excess mucus secretion, listlessness, clamped or tattered fins, flared gills, and small blood spots on the fins and body. Like most fish diseases, fluke outbreaks usually result from stress caused by poor water quality, improper diet, overcrowding, or aggression from other fish. They can also be introduced into your aquarium by new fish purchases that are not quarantined. The safest and most effective treatment for fish flukes is Praziquantel. Treating the aquarium with formalin can also be effective.

Lateral Line Disease (AKA: Head and Lateral Line Erosion or HLLE)

Also known as “hole in the head” this disease is caused by the protozoan Hexamita and is most commonly seen in the discus, Oscars, serums, and other cichlids such as angelfish, although it can infect any freshwater fish. Typical symptoms are cavities in the fish’s temples and along the lateral line. Another symptom is white, stringy feces. Hexamita often starts in the fish’s intestines and then spreads. Poor water quality is often a contributing factor to this disease.  It is usually treated with metronidazole, also sold as Flagyl. The most effective treatment method is medicated food, but if the fish has stopped eating, metronidazole can be added directly to the aquarium.

Nematode Worms

Commonly referred to as roundworms, nematodes can have direct or indirect life cycles. Those with direct life cycles can be passed directly from one fish to another when the fish eat nematode eggs or larvae. Those with indirect life cycles need a secondary host such as an invertebrate or another fish to complete their life cycle before becoming infectious again.  Roundworms can be difficult to accurately diagnose, and symptoms can be similar to many other diseases. Common signs of roundworm infection include hemorrhaging of the body, bloating of the abdomen or the opposite – wasting away even though the fish is eating well – cysts or lumps on the body, or the actual worm protruding from the fish’s anus. Another symptom is white feces, although this can also be caused by internal bacterial infections or Hexamita (see above). Effective treatments include levamisole, metronidazole, or praziquantel.  Metronidazole and praziquantel are especially effective when used as foot soaks. Antibiotics such as nitrofurazone or erythromycin may also help prevent secondary bacterial infections.  Read all package directions before using any medications and avoid mixing different medications in the aquarium.


Sometimes referred to as Fin and Tail Rot, bacterial infections are the second-most common diseases aquarium fish experience after parasites. They often follow parasite infestations, abrasions, or physical injury, but can also be brought on by chronic exposure to poor water quality and/or poor diet. Removal of a fish’s protective mucous membrane or scales during netting is another common cause.

Bacterial infections manifest in many ways, but common signs include a white film on the fish’s body or fins, cloudy eyes, tattered fins, and hemorrhaging (bloody patches) or open sores (ulcers) on the body and mouth. Also, if you still see white spots on your fish after 5 or more days of Ich treatment, your fish may have a secondary bacterial infection where the parasites bored into their bodies.

Treating for bacterial infections can be tricky and should be done with care, as some antibiotics can disrupt your aquarium’s biological filter. In addition, unless you have access to an incubator and are knowledgeable in fish pathology, correctly diagnosing what specific bacteria have infected your fish is nearly impossible. That said, some trends do exist and certain medications are known to be effective in specific instances. Always consult an experienced aquarium professional before treating your fish for bacterial infections.


True fungal infections in fish are less common than parasites or bacteria. They typically appear as white cottony or “furry” growths on fish but can also be internal. They can be induced by substandard water quality, infected food, or open wounds, but there are many other causes. Although fungal infections are not generally contagious, infected fish should be treated immediately with anti-fungal medication, preferably in a quarantine aquarium.


Viruses are tiny organisms that invade the fish’s cells and begin to replicate. They can be difficult to diagnose because they often produce similar symptoms to many other fish diseases.  Viral infections are well documented in koi and goldfish as well as angelfish, but they can affect all aquarium fish. There are no known cures for viral infections. Fish that are suspected of having a viral infection should be removed from the aquarium promptly to prevent spreading to other fish. Treatment with antibiotics or anti-parasitic medications in a quarantine aquarium can be attempted in case the infection is a bacteria or parasite and not a virus.


Also known as “pinecone disease” or Malawi bloat, dropsy is more a set of symptoms than an actual disease. It can be caused by a virus or bacterial infection of the kidney that results in fluid build-up in the fish’s abdomen, causing the fish to swell and the scales to stand on end, giving it the appearance of a pinecone. Treatment is dependent on what the cause is. While dropsy is not always treatable, antibiotics or the use of Epsom Salt in a quarantine aquarium have been known to produce results. Infected fish should be removed from the aquarium to prevent transmission to other fish.

Disease outbreaks can be avoided by purchasing healthy fish, maintaining optimum water conditions, and providing your fish with a balanced, nutritious diet. Learn to recognize common symptoms and contact your local fish expert if you think your fish are getting sick.

Please join our newsletter, connect with us on Facebook or contact us for more information.

Posted by: Salvador Rose on Category: Blog