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How to Spot Fish Illnesses

Updated: Apr 10

The main struggle for a pet owner with fish, is that aside from not talking you also can’t hold them or closely inspect them the way you could with a land based pet like cats, dogs, rabbits and reptiles for example. Because of this we really have to keep an eye on our fish and get an understanding for how they are in everyday life. From this, we can then notice if anything seems off with them. If taken care of properly, as in fed regularly but not too much, tank maintenance kept up to date and basic things like that, disease and infections should be very few and far between. However, the world isn't perfect and even something such as not cleaning equipment fully or introducing a new fish to the tank can be the source of aquarium wide health issues. Let’s have a run through what you can keep your eye out for.





The first is anchor worms.

These are introduced into new aquariums by infected fish, typically from the pet store. Young anchor worms are very small crustaceans that burrow into your fish’s skin. They enter the muscles where they then begin to develop and release eggs before dying off. The eggs hatch, the new anchor worms need to eat to grow, so they eat your fish from the inside out. This either causes lots of damage which then becomes infected, leading to death, or they simply overpower the fish completely, also leading to death. They can be stopped though!


What to look out for is if your fish is scratching against objects repeatedly while they try to remove the worms themselves. Another indication is simply being able to see the worms, they’ll look like white/light green threads that are coming from your fish’s skin. You might notice inflamed areas with a central point on the fish’s skin too - where worms have already laid their eggs then died off and left the body.


The main method of treating these worms is by physically removing the parasites yourself and cleaning the wounds with an antiseptic, such as iodine. If the worms don’t come off very easily then instead try bathing your freshwater fish in a seawater bath (around 35ppt) for 5 minutes a day, each day until the parasite falls off.




Next up is body flukes, these are flatworms that are only around 1mm long, so spotting the actual worms here is pretty hard. Instead, keep your eye out for other physical signs that may be a symptom. For example, fish with body flukes will try to scratch against objects in the tank in the same way they will with anchor worms but obviously you won’t be able to spot the worms. Another tip is checking if there’s a layer of mucus covering the gills or the body, or if the gills are moving rapidly. Finally, another sign of body flukes in gills or fins that have been chewed on, or reddened skin around them.


Pale fish with drooping fins, rapid respiration and/or hollow bellies indicate more extensive infestation.


They’re caused by undesirable environmental conditions including poor water quality, overcrowding and/or stress by incompatible species. These situations create conditions that can lead to destructive outbreaks. Flukes are often present in aquariums but remain harmless under ideal conditions. So keeping on top of your aquarium maintenance and avoiding stressful conditions is a key to prevention, but once an outbreak occurs, prompt treatment is critical.


Treatment for flukes is tougher because you’ll have to invest in a product, rather than just having a saltwater bath each day. Our recommendations include Tetra Parasite Guard which contains praziquantel and is quite effective, simply use one tablet per 10 gallons of water and remove any activated carbonin the filtration system. Conduct a partial water change in 48 hours then repeat the treatment. Another option is Waterlife’s treatment for general parasites, which contains Sterazin instead. Always follow the manufacturer's guidelines on the back of the packaging and keep your eye out for secondary infections as a result of your fish’s injuries.



Clamped Fin - this ones fairly easy to spot, the fins will be folded against the body instead of being fanned out as they usually are. This one’s trickier because its not indicative of any specific illness or disease, it can be a reflection of various problems including parasites or bad water quality. It’s important to first determine the cause and then you can work out a solution. Start by cleaning the tank and boosting the water quality, if it persists then it’s likely a parasite problem. A good first line of treatment is a multipurpose antibiotic such as Tetra Lifeguard which covers a range of bases. You can also add 1 tbs of aquarium salt per gallon of water in your tank, which helps to prevent clamped fin. Again, before you use antibiotics, remove any activated carbon you may have in the tank's filtration system.


Dropsy is caused by a bacterial infection of the kidneys. It causes fluid accumulation or renal failure but it appears to create problems only in weakened fish, this suggests it may come from unfavourable aquarium water conditions. You can pick up on dropsy by noticing any bloating in your fish or protruding scales on their bodies.


Again for treatment we recommend you use Tetra Lifeguard and remove and carbon from your filters as well as turning off UV sterilizers. Use one tablet per 5 gallons of water and treat for 5 consecutive days. Preventative care can be given too such as water changes and maintaining good aquarium chemistry. You can also add aquarium salt to the water too boost chances of survival.


Fish ick is caused by rapid temperature and/or pH fluctuations and creates spots on the fish’s skin which resembles sand or grains of salt. This skin is irritated so the fish may be brushing up against ornaments in the tank in an attempt to scratch their itch. They may also have clamped fins that are tucked in against the body rather than being fanned out as they usually are and the fish may be gasping at the water's surface.


Again we turn to products such as Tetra Lifeguard to save these sick fish. Turn off any UV sterilizers you have and remove any carbon from the filtration systems. Add one tablet per day to each 5 gallons of water. Treat for 5 consecutive days. Repeat until symptoms clear. Secondary infections are also common and can be treated with antibiotics or general cures. Consistent temperature and good water quality will help prevent infections, in addition to using aquarium salt.


Fungus can be spotted as a gray or white growth in and on the skin or fins of your fish. Untreated fungus appears as a sort of cottony growth and eventually, this fungus continues to eat your fish’s body in order to grow and your fish will die from it. Fish that tend to develop fungus issues are usually already weak or vulnerable. This may be the result of another illness or health issues such as parasites or a physical injury or infection.


Again, a catch all solution is Tetra Lifeguard.

Remove the carbon from your filter set up and turn off the UV sterilizer if you have one. Add one tablet per day to each 5 gallons of water. Treat for 5 consecutive days and repeat until symptoms are clear. Many bacterial infections are misdiagnosed as fungal, so common medications include both a fungicide and antibiotics. Tetra Fungus Guard contains malachite green and formalin; treat one tablet per 10 gallons once every 4 days until symptoms are gone. Remove activated carbon and conduct partial water changes in between treatments.


Gill flukes, like body flukes, are flatworms which are too small to see. Instead, keep your eye out for infected gills and skin and keep a close eye on them, as once their gills are too far gone, they’ll suffocate and die. It’s caused by bad environmental conditions such as poor water quality, overcrowding and/or stress by incompatible species. These environmental factors create conditions that can lead to destructive outbreaks. Flukes are often present in aquariums but remain harmless under ideal conditions. Avoiding stressful conditions is a key to prevention, but once an outbreak occurs, prompt treatment is critical.


Our recommendations include Tetra Parasite Guard for treatment which contains praziquantel and is quite effective, simply use one tablet per 10 gallons of water and remove any activated carbonin the filtration system. Conduct a partial water change in 48 hours then repeat the treatment. Another option is Waterlife’s treatment for general parasites, which contains Sterazin instead. Always follow the manufacturer's guidelines on the back of the packaging and keep your eye out for secondary infections as a result of your fish’s injuries.


Gill mites are brought into the tank by fish that are already infected, they then spread to the other fish. Mites are absolutely tiny but you may see them if you have a magnifying glass. You’ll notice them around the gill covers and your fish will most likely be gasping for air at the water's surface. The mites stay on the fish’s gills, and attack the fish by feeding on blood and living flesh.


Remove filter carbon and turn off UV sterilizer. Add one tablet or Tetra Lifeguard per day to each 5 gallons of water. Treat for 5 consecutive days. Conduct a partial water change between treatments (after the 5 consecutive days - do not change the water daily as this will add far more stress to your fish than is recommended). Secondary infections are also common and can be treated with antibiotics or general cures like Tetra Fungus Guard.


For Hemorrhagic Septicemia a variety of different symptoms may occur, though some fish exhibit no external symptoms. Symptoms include hemorrhaging of internal organs, skin and muscle, bulging eyes and bloated abdomens, a general bruised appearance with reddish tints to eyes, skin, gills and fins, open sores or simply abnormal behavior.


The cause of this is an infection that is brought into aquariums by fish already infected with a deadly virus called Viral hemorrhagic septicemia virus (VHSV or VHSv). Unfortunately there is no known cure for this virus. Sometimes treating secondary infections with wide spectrum antibiotics or general use treatments like Tetra Lifeguard will reduce mortality. Add one tablet Lifeguard to each 5 gallons and treat 5 consecutive days. Remove activated carbon and conduct a partial water change in between treatments.




Symptoms of lice in fish include an aggravated or restless mood. They may also rub up against objects and aquarium glass in an attempt to remove the lice. You may be able to spot them as they look like tiny pale crabs. They appear as flat, dark oval dots crawling on your fish. They feed on the host by using suckers to attach to the fish then pierce the skin.


Usually a lice infestation comes from fish that were living in an outdoor pond at one time and bringing them into an indoor aquarium. Also, fish lice can be introduced from wild fish that are added to an aquarium. Lice travel from one host fish to another, spreading bacteria and viruses, so once they’re in your aquarium, you must get rid of them.


Common methods or treatment include physically removing the parasite and cleaning the wound with an antiseptic like iodine, as well as bathing freshwater fish in a seawater bath (35ppt) for about 5 minutes for multiple days until the parasite falls off, or using a formalin bath.


Ragged tail fin or “tail, fin and mouth rot” is the progressive deterioration of the tail and the fins. Fins will become frayed and their colour will fade. It’s caused by a bacterial infection in susceptible fish which are typically those who are bullied by other fish or have been injured by fin nipping tank mates, especially when water quality is poor.


Remove filter carbon and turn off UV sterilizer. Add one tablet or Tetra Lifeguard per day to each 5 gallons of water. Treat for 5 consecutive days. Conduct a partial water change between treatments (after the 5 consecutive days - do not change the water daily as this will add far more stress to your fish than is recommended). Secondary infections are also common and can be treated with antibiotics or general cures like Tetra Fungus Guard.


As you can see, Tetra Lifeguard is a very handy product and will treat a wide range of health issues in your tank and it’s one of the products we can fully recommend. We recommend buying a bottle (you can buy it here) just in case it’s needed and keeping it alongside your water treatment products. It’s a lot better to have it on hand when you discover that you need it, rather than having to wait a week for delivery and risk losing your tank mates before you can intervene.



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