It’s a fine lazy day and you’re just kicking back and enjoying the aquarium you’ve worked so hard to set up. But wait, something’s off. One of your fish looks like he’s been rolling in the sand. And another seems to have less fins than you remember. And still another is so swollen he looks as though he’s about to burst. All in all, it looks as though your fish are sick! Yup, keep fish long enough and it’s something you’ll have to face eventually, and usually fairly early unfortunately. You see, illness is often preventable, but typically we only learn how to do so after doing it wrong the first time. But fear not! Many ailments can be turned around if spotted early and treated properly.

Spotting Illness

So how can you tell that a fish is sick in the first place? It’s not like they’re going to tap you on the shoulder and let you know. For the most part the only way you’ll know something is wrong is through careful observation of their appearance and behavior. Hopefully you’re already fairly well acquainted with what could be considered normal for your fish and can thus notice when something is off. Here are some key things to take note of:

-paleness/color change

-clamped fins (the fins are held close to the body)

-scraping or rubbing against objects in the tank

-heavy breathing

-loss of appetite

-loss of equilibrium


-decreased activity

… and of course the more obvious signs like visible sores, swelling, and the like.

Keeping a watchful eye out for signs of illness is an important part of keeping fish. Try to take a few minutes each day to check for any signs that something is amiss. Feeding time provides an ideal opportunity to do this as most fish are at their most active when there’s a meal to be had. An illness caught early is far easier to treat and the chances of the affected fish surviving the ordeal are far greater. For many ailments your fish may face by the time it’s blatantly obvious it is too late.

Quarantine Tanks

Of course one step better than treating your fish once they become sick is preventing it from happening in the first place. The absolute best way to prevent diseases from reaching your tank is by using a quarantine tank. A quarantine tank is essentially just a small bare bones aquarium setup where all new arrivals can spend a week or two before entering your main setup. This gives you ample time to make sure your new fish are in good health before they have a chance to potentially spread any diseases to your other fish. It also gives new arrivals a chance to recover from the stress of moving in a quiet and peaceful environment. And if a problem does arise having the specimen already isolated makes treatment much easier as well. Finally, in the event that a problem does reach the fish in the main aquarium the quarantine tank can serve as a hospital tank as well, preventing the further spread of disease and providing a safer and more controlled environment for the application of any treatments.

Water Quality

In addition to a quarantine tank, keeping your fish in good general health goes a long way towards preventing any illness from taking hold. Most common diseases often arise in fish only when their health is already compromised. What causes their health to become compromised? The majority of the time the culprit is poor water quality. A fish trying to live in dirty water in kind of like you trying to live in a house filled with smoke- it’s unlikely you’ll be in the best of health. Keeping on top of your aquarium setup’s maintenance is key to keeping your fish healthy and disease free. As such, should your fish ever become ill your first step should always be to make sure the water is in excellent condition. All the critical parameters, such as ammonia, nitrate, pH, and temperature, should be checked. Always be suspicious of any equipment or decor that was recently added to the tank as well which could be leeching something toxic into the water. And, even if poor water quality isn’t the root cause of the illness, a water change is never a bad idea When it comes to recovery the cleaner the water the better.

About Medications

Often people go straight for the medications at the first sign of an illness in their fish, usually without even knowing what exactly is wrong. This is a bad move. Positive identification of a disease is absolutely essential before beginning application of any medication. Many medications aren’t exactly easy on your fish either meaning using the wrong one could end up further stressing your fish without curing their illness, likely leading to death. Still, should you encounter a disease where a medication is applicable it can be a real life saver. Just make sure to remove any carbon from the filter before beginning treatment as it will soak up the medication before it has a chance to act. And, it should go without saying that the directions should be followed to a T. Pay particular attention to any warnings dealing with species the medication should not be used with. Some, for example, will kills snails and plants if there are any in the tank.

Common Ailments

There are tons and tons of diseases your fish may face- far more than what are listed here. However, many of them are fairly rare, affecting only a few specific species or only arising under specific circumstances. Instead, this list tries to cover only the most common ones that most aquarists tend to run into.

Ammonia Poisoning/New Tank Syndrome

signs- red irritated gills, fish gasping for air at the surface, most common in new tanks

As the common name of ‘new tank syndrome’ suggests, this is typically only a problem in freshly setup aquariums, although it can occur is older systems if the filtration system is severely damaged. Basically, not all bacteria are out to make a meal of your fish. Some are actually quite helpful, and necessary, to your aquarium. Their job is to process the fish’s waste from highly toxic substances, namely ammonia, into compounds they can more easily tolerate. The process of establishing these bacteria in a new system is called cycling. Unfortunately, this step is frequently skipped leading to a buildup of ammonia which in short order leads to dead fish.

treatment: water changes

To solve this problem you basically just have to keep the water clean through frequent water changes until the bacteria have established themselves and can take over. A test kit for ammonia is very helpful here as ideally you want to keep the ammonia level under 1ppm. Typically you will need to do a small water change daily for a few weeks to allow the cycle to complete while keeping the tank habitable for your fish. Keeping feedings light during this time can also help keep the waste load low which in turn keeps the water cleaner.

Fish Fungus

signs- fish has fuzzy whitish globs or patches attached to its fins and/or body

Like a lot a ailments you’re likely to encounter, fish fungus usually appears on fish whose health has already been compromised. The fungi that cause it are quite common and pretty much guaranteed to be in the tank at all times. This normally wouldn’t be a problem, but when a fish is already in poor shape the fungus can get a hold very easily. It often starts at the site of an injury, which could be anything from small scrapes to major sores, and spreads quickly from there.

treatment: medication for fungal infections

The best way to treat fungal infections is with medication designed for them. Of course making sure the tank is in good shape is key as well as the fish is likely already in poor shape and needs all the help he can get recovering. As a side note, many remedies for fungal infections also work against bacterial infections which can bring some added benefit if the fish’s initial poor health was brought on by one.

Ick/Ich and Velvet

signs- fish is covered in lightly colored specks or has a dusty appearance


Ask people to name a common fish disease and ick is probably the one you’ll get. It seems just about everyone who’s ever kept fish has had to deal with it at one point or another. Furthermore, it’s quite easy to spot compared to other diseases and so seems to stick with people. If you haven’t encountered it before, ick is a parasite that burrows into the fish’s skin causing little white spots that make it appear as though your fish has been salted. Outbreaks often occur after the addition of new fish, which bring ick along with them, although it can also limp along in a tank for a long time until conditions are favorable for a explosion. Like pretty much every disease, favorable conditions means fish in poor health with the most common reason being poor water quality.


Velvet is less common but still worth mentioning. It’s another parasite that behaves much like ick, appearing as spots on the skin. The difference is the spots are much smaller and may have a yellow to greyish appearance. With enough of them it can sort of blend together giving the fish a fuzzy velvety appearance, hence the name.

treatment: medication for parasites

Treatment for both ick and velvet is pretty much the same. They’re both parasites with a similar life cycle- part of which is spent attached to a fish and part of which is spent free swimming. Killing them is more or less impossible while they’re safely burrowed under the fish’s skin. It’s only when they emerge into the open water to look for a new host that they’re vulnerable. This means treatment can take some time. Medication needs to be applied for an extended period to basically wait out the parasite’s natural life cycle, which can take up to a month. Raising the temperature of the aquarium a few degrees can help speed things up a bit. Also one other note- these parasites do require a fish host to complete their life cycle. So, should you move all your fish into quarantine for treatment, any parasites left in the main tank will die off after about a month.


signs- fish is bloated, possibly with the scales protruding giving it a pine cone appearance

Dropsy isn’t a disease itself but rather the physical outcome of some other ailment, usually a bacterial infection although it could be caused by any number of other things. The swelling is brought on by a buildup of fluid in the fish’s body cavity.

treatment: medication for bacterial infections, aquarium salt

Unfortunately by the time the symptoms are clearly visible it’s often too late to save the fish. Still, the best course of action is to move the fish to quarantine and begin administering an antibiotic, preferably in the form of an medicated food. Adding a small amount of aquarium salt may also help the fish expel some of the excess fluid thus alleviating the swelling. Use around one tablespoon per five gallons.

Swim Bladder

signs- fish has difficulty maintaining equilibrium and may have trouble controlling depth

A fish’s swim bladder is sort of like a ballast tank in a submarine, only with air instead of water. They use it to keep themselves upright and at the correct depth. When it becomes damaged or otherwise perturbed the fish is no longer able to control this air and so will usually either sink to the bottom or float to the top, often in an off-kilter orientation.

treatment: fasting/cooked peas, medication for bacterial infections

Unfortunately there’s not a lot of consensus on the exact cause (and in fact there are at least a couple possible causes) or how to treat it. Making sure the tank’s chemistry is in good shape should be your first step of course, as it should be with any problem. One common cause is a blockage in the fish’s digestive system. The common solution for this is shelled cooked peas (they’re sorta the go-to fish laxative). Backing off on feedings for a few days can also help. Try this and see if the problem clears up. If not then it may be the symptom of a bigger infection in which case a medication can be tried. Unfortunately it can also sometimes arise due to trauma sustained during transport in which case there’s not much that can be done. In these cases all you can do is give the fish a place to recover and hope for the best.

Pop Eye

signs- fish has a protruding eye

A pretty self explanatory name, the fish’s eye or eyes bulge out from the fish’s head as if they’re on the verge of falling out completely. It’s basically an inflammation of the eye causing it to swell and protrude. Once again this isn’t so much linked to any one specific cause but instead may originate from a couple possible sources, namely injury or an infection.

treatment: improve conditions, medication for bacterial infections, aquarium salt

Injury due to fighting or possibly from bumping into something is probably the most common. Think of it like the fish equivalent of a black eye. A good indicator that this is a the cause is if only one eye is affected. In this case the best you can do is stop any fighting and provide your fish with a peaceful home in which to recover.

Another possibility is a bacterial infection. If both eyes are popped then this is more likely, though the fish should still be checked for signs of injury or fighting. A good medication is the best course of action here.

Finally, another thing you may try is aquarium salts. The increased salinity will help draw out the excess fluid, relieving the swelling. Add one tablespoon per five gallons and watch to see if it has any effect. Remember this can help relieve the swelling but won’t fix the underlying problem and as always your first step should be to make sure the water is in good shape.

Cloudy Eye

signs- fish has a cloudy eye

The creatures of the world have all sorts of interesting eye with various colors and shapes and whatnot. One thing they all have in common, though, is that the center is nice and clear. If you ever notice your fish’s eye or eyes becoming milky then something is not quite right.

treatment: improve conditions, medication for bacterial infections

There are a handful reasons a fish’s eye may become cloudy. Parasites or bacteria are possible causes, particularly if the eye has been injured. Poor diet or even cataracts as a result of old age are possible culprits as well. But, the most likely cause falls in line with so many other problems- poor water quality. Getting the tank is good shape should be your first concern if your fish develops cloudy eyes. Pay particular attention to the pH as an especially low pH is thought to contribute to this problem. Check that you are feeding your fish an appropriate diet as well. With better water quality and a proper diet the problem should clear in a few weeks. However, if the problem does not clear after several weeks with improved conditions an antibiotic can be tried.

Hole in the Head/Lateral Line Disease

signs- fish has holes in its head

The name is pretty self explanatory, and the symptoms are easy to spot, so all that leaves is the cause. Why are there unwelcome holes in your fish’s head? Unfortunately there’s no consensus on the exact cause. It may be caused by some specific pathogen, but none has been positively identified as of yet. Some think overuse of activated carbon or nutritional deficiencies may have something to do with it as well.

treatment: improve conditions, diet change

Ultimately, your best bet is to improve conditions as much as you can for your fish. Step up the water changes and try removing any activated carbon from your filtration. Try to add as much variety to your fish’s diet as you can. Frozen as well as vitamin enriched flake foods are great sources of vital nutrients your fish may be lacking.

Fin Rot

signs- the fish’s fins are deteriorating

Fin rot is another ailment that’s not so much about a specific contagion rather than a result of the fish’s overall health. Fin rot typically only affects fish that are already stressed or weak due to something else such as poor water quality, malnutrition, bullying, or possibly even some other sickness, at which point bacteria move in and start feasting on your poor fish’s fins.

treatment: improve conditions, medication for bacterial infections

Your first course of action should be to determine why the fish’s health has slipped in the first place and correct it. Check the water quality and switch to high quality vitamin enriched foods if you haven’t already. An antibiotic may be necessary if the damage is severe (more than just a small section of the fin).

Again, the majority of the time health issues come down to a water quality problem. Keeping on top of your aquarium maintenance schedule is the best way to prevent problems in the first place. Likewise, if an illness should strike checking the water’s parameters should be your first step. Many problems will clear on their own with improved conditions. And if a medication becomes necessary make certain to follow the directions extremely closely. An overdose of medicine can be just as bad if not worse than the illness itself.