What's new

What to do if your fish gets sick


Jan 26, 2008
Reaction score
Perth, WA
If your fish ever looks sick or unwell, then the following steps might help.

Test the Water and Clean the Tank.
Test the water quality for ammonia, nitrite, nitrate and pH and write the results down in numbers. Check it for general hardness (GH) and carbonate hardness (KH) too if you can, but these two items are not normally a cause of problems, however they are good to know. Again write the results down in numbers.

If you don't have test kits then take out a glass full of tank water and put a lid on the glass and put it to one side. Make sure nobody tips this water sample out. You can take this water sample to a pet shop for testing after reading this information and taking the following steps.

When you get water tested at a pet shop, write down the results (in numbers) when they do the tests. If the shop says the water is good, ask them what the results were in numbers and write them down. If getting water tested for hardness, ask the shop if the results are in ppm or dGH. They have different results.

Once you have tested the water or taken a sample and set it aside for testing later, wipe the inside of the glass down, and do a 75% water change and gravel clean the substrate each day until the issue has been identified.
*NB* Make sure any new water is free of chlorine/ chloramine before it is added to the aquarium.

Clean the filter materials in a bucket of tank water if it hasn't been done in the last 2 weeks. If the tank has been set up for less than 6 weeks, then do not clean the filter because you can remove the beneficial filter bacteria that is growing there. However, if the tank has been set up and running for more than 8 weeks, then clean the filter if it hasn't been done in the last 2 weeks.

Most fish health issues are caused by a dirty environment or poor water quality. Doing a big (75%) water change each day will dilute disease organisms and reduce harmful chemicals/ substances in the water. Big daily water changes and gravel cleaning the substrate will clean the environment up and buy you some time to find out what is causing the problem. If the issue is ammonia, nitrite or nitrate, then the big daily water changes will normally fix the problem within a couple of days.

Ammonia & Nitrite problems.
If you have an ammonia or nitrite problem in the tank, reduce the feeding to once every couple of days.

Ammonia is produced by anything that rots or breaks down in the water. This can be fish food, fish waste, dead fish, dead plants or even rotting driftwood, although driftwood is normally safe and only rarely causes issues in aquariums. The less rotting material in the tank, and the less food going into the tank, the less ammonia that is produced. Less ammonia means less nitrite and less nitrate in the water.

If you reduce the feeding to once every couple of days, Do Not worry about the fish starving. Unlike mammals and terrestrial animals that use most of the food they eat to keep warm, most fish take their body temperature from the surrounding water. This means any food they eat is used for growing and swimming. Because of this, fish can go for weeks or even months without food and not die from starvation.

Most aquarium fish are over weight and most water quality issues are caused by too much food going into the tank, and not enough water changes being done to compensate for the food. So if there is a water quality issue (caused by ammonia, nitrite or nitrate) reduce the feeding and do big daily water changes.

Post Pictures on a New Thread.
Once you have cleaned the filter and done the water change & gravel clean, look at the fish. See if they have any unusual patches, spots, marks or discolouration on them. Get a digital camera or mobile phone and take some pictures of the fish and check the pictures on your computer. Pick out a couple of good clear pictures that clearly show the problem and post them on the forum for us to look at. You will obviously have to start a new thread in the emergency section of the forum to do this, so start one at the following link.

If the website says the images are too big, set your camera's resolution to its lowest setting and take some more pictures, then try again. If the images are still too big you can use image editing software to crop them down, or post them on an image hosting website and put the link to the pictures here on the forum. We can use the link to view the images at the other website.

When photographing fish, open the curtains (if its day time), turn the room lights on and have the tank light on. Use the camera flash too. Cameras need light to get good images so the more light the better the pictures.

At the bottom of the emergency link is a button called "Post New Thread". Click that button and you will be able to start your new thread.

At the top of the emergency link is a thread that has been pinned. It is called "Start Here With Your Emergency". Copy and paste the questions from that thread into your new thread and answer the questions to the best of your ability. Add the good pictures and submit the thread.

For New Members.
If you are a new member to this forum and have not yet posted anything, start a thread saying Hi and let that get posted. It can take up to 24 hours for your first post to be published online and if you have a fish health issue, 24 hours can be too late. By making your first post as soon as you join the forum, you get it out of the way and then if you have a fish emergency, you can post online immediately and get help sooner.

So don't just sit back and read the articles, make your first post in the introduction section (see following link) as soon as you join. It doesn't have to be anything special, just "Hi, I'm so and so and I keep a few fish", or "I am interested in keeping fish" if you don't have any currently. Then go read the articles.

Test Kits & Water Testing at Pet Shops.
If you don't have test kits, then once you have done the water change, gravel and filter clean, taken pictures and posted a new thread in the emergency section of the forum, take the water sample to the local pet shop and ask them to test the water for you. Some shops charge a small fee for testing. This is to cover their costs for the test kits. Other shops do it for free. Check with the shop first.

If you want to buy test kits then try to get liquid test kits rather than dry paper strip test kits. The liquid kits are generally more accurate.

You can either buy individual test kits for ammonia, nitrite, nitrate, pH, GH & KH. Or you can buy a master test kit that contains some or all of these. Check the price and see what tests are included in master test kits. It is sometimes cheaper to buy the individual kits but not always. If the master kit does not have an ammonia test, then you will have to buy that separately.

Ammonia (NH3/NH4), Nitrite (NO2), Nitrate (NO3), and pH are the 4 main test kits you need for fish keeping. GH & KH should be tested initially when first starting out to see how much mineral content is in the water. Then you can check GH & KH every 3-6 months after that. Most people just get their local shop to test GH & KH a couple of times a year but buy the other test kits so they can monitor the 4 main levels in the water.

Check the packaging and make sure the test kits have not expired and are not going to expire in the next few months.

Try to get test kits from shops that keep them in a cool dry place. Heat causes the reagents (chemicals) in the kits to break down so if the shop has the test kits in front of a window where the sun shines in, or near a heater or in a fish room, avoid getting kits from there. The same applies to medications. Only get them if they are kept in an air conditioned/ unheated showroom and they are not near any heat source.

When you get the kits (or medications) home, put them in a plastic container and keep them cool and dry. I kept mine in a plastic icecream container with a lid on the bottom shelf in my fridge.

Make sure children and animals cannot get these test kits or medications because they contain poisonous chemicals.

Diseases & Water Quality.
Once you have done the above you can try to work out what is going on with the fish. If the water tests were good and there was 0 ammonia, 0 nitrite and less than 20ppm nitrate, and the pH was in a range suitable for the fish (generally between 6.6 & 7.8 for most fresh water fishes), then the fish might have a disease. However, if there was an ammonia or nitrite reading above 0, or the nitrate reading was above 40ppm, then water quality is definitely an issue and doing big daily water changes and gravel cleaning the substrate, will help.

All fish have a thin clear mucous layer coating their body and fins. This helps reduce drag in the water and is a first line of defence against things that might irritate the skin. Fish kept in poor water quality or fish that are being attacked by parasites will produce more of this mucous to try and reduce irritation and prevent infection.

Fish that produce a cream or white film over their body and fins are just about always in poor water quality. That is to say there is usually ammonia, nitrite or high nitrate levels in the water, or the pH is unsuitable for the fish. Heavy metals and other chemicals in the water will also cause this but 99% of the time it is poor water quality caused by ammonia, nitrite, nitrate or pH. The fish get stressed from the toxins in the water and produce more mucous to protect them from what's in the water.

If the fish get cream, white or grey patches on their body, it is usually a protozoan infection caused by poor water quality, (see "Protozoan infections" below).

If your fish ever get a cream or white film over their body, fins and eyes, do a big (75%) water change, gravel clean the substrate, clean the filter and increase aeration immediately. Do the big water change and gravel clean each day until the problem is identified or resolved.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.

Big water changes will not affect beneficial filter bacteria or fish as long as the new water is free of chlorine/ chloramine before it is added to the tank.

Regarding diseases, most fish diseases fall into one of three categories, bacterial, fungal or protozoan. There are also viruses and a few other diseases that turn up occasionally that do not fit in these 3 main categories, but they are uncommon.

Bacterial infections usually appear as red areas on the body or fins and do not normally occur in clean aquariums or on healthy fish. The harmful bacteria gain entry into a fish through damaged tissue, which is normally caused by fighting or poor water quality.

Most bacterial infections in fish are introduced into the aquarium by diseased/ contaminated fish. Quarantining all new fish for at least 4 weeks is a good idea and prevents most unwanted diseases getting into your display aquarium.

Most bacterial infections in fish can be treated with a broad spectrum fish medication and Do Not require anti-biotics. Anti-biotics should only be used as a last resort for known bacterial infections that do not respond to normal fish medications. Improper use or mis-use of anti-biotics can lead to drug resistant bacteria that can harm or kill people, animals and fish. So anti-biotics should only be used if you know what the issue is and have tried other types of medications.

Fungal infections in fish normally appear as white fluffy patches on the body or fins.

The fungus gets into damaged tissue, similar to the way bacteria does. If fish are kept in a clean environment and are not damaged physically, they should never get fungal infections.

If fish do get fungal infections, it is normally just one fish that develops it and it is normally on an injured part of the body.

There are numerous medications designed to treat fungus on fish. Methylene Blue is commonly used, however this kills filter bacteria and stains silicon blue. Silicon is the glue used to hold glass aquariums together. Staining the silicon blue does not harm the glue but does make it look odd.

If using a medication containing Methylene Blue, try to treat the fish in a separate quarantine tank so you don't damage the filter in the main tank. Methylene Blue also treats minor bacterial infections in fish.

Protozoan infections are the most commonly encountered diseases in aquariums. They are generally found in all water bodies (fresh, brackish and salt water), and thrive in dirty aquariums with lots of rotting organic matter, and aquariums that don't get enough water changes.

The most well known protozoan infection is whitespot (Ichthyophthirius), which produces small white spots/ dots on the body and fins. Healthy fish catch this parasite when the disease organism is introduced into the aquarium on a contaminated fish or plant, or in contaminated water. There is more information about whitespot and treating it at the following link, post #17 (2nd post on page 2).

Less well known are Costia, Chilodonella & Trichodina, which are regularly found in aquariums and cause cream, grey or white patches on the fish's body. In severe cases there will be red around the edge of the cream, grey or white patch and this is blood leaking out of the fish where the tiny parasites are eating it.

Velvet (Oodinium) is another protozoan parasite that produces a gold sheen on the fish. If you turn the tank lights out and shine a torch on a fish with velvet, it should appear as if the fish has a gold sheen over part or all of its body. This is most noticeable on dark coloured fish like black mollies, but can be seen on most infected fish.

All protozoan parasites that live on the outside of the fish cause irritation and most infected fish will rub on objects in the tank. Poor water quality can also irritate the fish and cause them to rub on things and fish sometimes get an occasional itch, but if some or all of the fish in an aquarium are regularly rubbing on objects, they probably have a protozoan infection.

All external protozoan infections can be treated with any white spot remedy, or with salt and or heat. See post #2 (below) for information on using salt to treat some fish diseases. Heat treatment simply involves raising the tank water temperature to 30C and holding it there for 2 weeks.

*NB* Heat should not be used to treat cold water fishes unless they are already in warm water (above 24C), like during summer.

Regardless of if you use medications or heat to treat fish, you should clean the tank first (do a big 75% water change, gravel clean the substrate, wipe the inside of the glass, and clean the filter). Then increase aeration/ surface turbulence to maximise the oxygen levels in the water. Most chemicals and salt reduce the oxygen levels in water, and warm water holds less oxygen that cool water.

Cleaning the tank before treating it will reduce the gunk and harmful disease organisms in the water, gravel and filter, and this will allow the medication to work on the sick fish rather than on the pathogens living in the rubbish in the tank.

Never use medications unless you know what the problem is.
If in doubt, water change it out. Water change and clean the tank first, medicate later if needed.
Last edited:


Jan 26, 2008
Reaction score
Perth, WA
If the mods want to sticky this feel free to do so.

For anyone who wants to read it, print it out and read it in bed. :)

Using Salt to Treat Fish Health Issues.
For some fish diseases you can use salt (sodium chloride) to treat the ailment rather than using a chemical based medication. Salt is relatively safe and is regularly used in the aquaculture industry to treat food fish for diseases. Salt has been successfully used to treat minor fungal and bacterial infections, as well as a number of external protozoan infections. Salt alone will not treat whitespot (Ichthyophthirius) or Velvet (Oodinium) but will treat most other types of protozoan infections in freshwater fishes.

You can add rock salt (often sold as aquarium salt), sea salt or swimming pool salt to the aquarium at the dose rate of 1 heaped tablespoon per 20 litres of water. If there is no improvement after 48 hours you can double that dose rate so there is 2 heaped tablespoons of salt per 20 litres.

If you only have livebearers (guppies, platies, swordtails, mollies), goldfish or rainbowfish in the tank you can double that dose rate, so you would add 2 heaped tablespoons per 20 litres and if there is no improvement after 48 hours, then increase it so there is a total of 4 heaped tablespoons of salt per 20 litres.

Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in fish from soft water (tetras, Corydoras, angelfish, gouramis, loaches) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers, rainbowfish or other salt tolerant species.

The salt will not affect the beneficial filter bacteria but the higher dose rate will affect some plants. The lower dose rate will not affect plants.

After you use salt and the fish have recovered, you do a 10% water change each day for a week. Then do a 20% water change each day for a week. Then you can do bigger water changes after that.

General dose rates for salt (sodium chloride) to treat tetras, barbs, danios, rasboras, angelfish, discus, Corydoras and loaches is 1 heaped tablespoon per 20 litres of water. If there is no improvement after 48 hours you can increase the salt level to a total of 2 heaped tablespoons per 20 litres of water.

If you are treating livebearers, goldfish, rainbowfishes, glassfish (Chandas) and certain other fishes that are tolerant of brackish (salty) conditions, you can use 2 heaped tablespoons of salt per 20 litres, and if there is no improvement after 48 hours you double that so there is a total of 4 heaped tablespoons of salt per 20 litres of water.
Last edited:


Oct 7, 2013
Reaction score
Very useful article, especially for beginners
In particular the warning about Identifying the problem before treating it.
I feel that a lot of harm can be done by using the incorrect medication with good intentions.
Last edited:
Search tags for this page

using salt for sick severum