Freshwater Fish Disease – Signs, Causes, And Cures

Smile Fish Aquarium Open Heart Happy Fish

It’s a fine lazy day and you are just kicking back and enjoying the aquarium you’ve worked so hard to prepare. 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 bloated he looks like 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 that you’ll have to face eventually, and usually fairly early sadly. You see, illness is often preventable, but we just learn how to do this after doing it wrong the first time. But fear not! Many ailments can be turned around if spotted early and treated properly.

So how do you tell that a fish is ill 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 behaviour. Hopefully you are already pretty well acquainted with what could be considered normal for your fish and can thus notice if something is off.

-paleness/color change

-clamped fins (the fins are held near the body)

-scraping or rubbing against objects in the tank

-heavy breathing

-loss of equilibrium


-decreased activity

… and of course the more obvious signs like visible swelling, Opossum Poop, nausea, and the like.

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

Quarantine Tanks

Obviously one step better than treating your fish as soon as they get sick is preventing it from occurring in the first location. 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 set up where new arrivals can spend a week or two before entering your main setup. This gives you ample time to make sure that your new fish are in good health before they have a chance to potentially spread any diseases to your other fish. Additionally, it gives new arrivals a chance to get over the stress of moving in a quiet and serene atmosphere. And if a problem does arise with the shredder already isolated makes treatment much easier also. Finally, in the event that a problem does reach the fish in the home aquarium the septic tank can serve as a hospital tank also, preventing the further spread of illness and providing a safer and more controlled environment for the application of any treatments.

Water Quality

Besides a quarantine tank, maintaining your fish in good general health goes a long way towards preventing any illness from taking hold. Most frequent ailments often arise in fish only when their health is already compromised. What causes their health to become compromised? The majority of the time the offender 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’re going to be in the best of health. Keeping on top of your aquarium installation’s maintenance is key to keeping your fish healthy and disease free. As such, if your fish ever become ill your first step should be to make sure that the water is in excellent condition. All the vital parameters, such as ammonia, nitrate, pH, and temperature, should be checked. Always be leery of any equipment or decoration that was recently added to the tank as well which could be leeching something toxic into the water. Andif poor water quality is not the origin of the illness, a water change is never a bad idea once it comes to recovery the cleaner the water the better.

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

Common Ailments

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

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 when the filtration system is badly damaged. Basically, not all bacteria are out to make a meal of your fish. Some are really quite useful, and necessary, to your aquarium. Their task is to process the fish’s waste from highly toxic substances, namely ammonia, to compounds they can more easily tolerate. The process of establishing these bacteria in a new system is called cycling. Unfortunately, this measure is often skipped leading to a buildup of ammonia that in short order leads to dead fish.

Treatment: water changes

To solve this issue you basically just have to keep the water clean through regular water changes before the bacteria have established themselves and may take over. A test kit for ammonia is quite helpful here as ideally you would like to keep the ammonia level under 1ppm. Typically you’ll need to do a small water change daily for a couple of weeks to allow the cycle to complete while keeping the tank habitable to 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

Signals – fish has fuzzy whitish globs or stains attached to its fins and/or body

Just like a lot a ailments you’re going to experience, fish fungus usually appears on fish whose health is already compromised. This normally wouldn’t be a issue, but if a fish is already in poor shape the fungus can get a hold very easily. It often begins at the website of an accident, which could be anything from minor scrapes to significant sores, and spreads quickly from there.

Treatment: medication for fungal infections

The best way to treat fungal infections is with medication designed for them. Obviously making sure the tank is in great shape is key and the fish is probably 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 could bring some extra benefit if the fish’s initial poor health was caused by one.

Ick/Ich and Velvet

Signs- fish is coated in lightly colored specks or has a dusty look


Ask people to name a common fish disease and ick is most likely the one that you’ll get. It seems just about everyone who’s ever maintained fish has had to handle it at one point or another. What’s more, it’s quite easy to see 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 small white spots that make it appear as though your fish was salted. Outbreaks often occur after the accession of fresh fish, which attract ick and them, although it can also limp along in a tank for a long time until conditions are favorable for a explosion. Like nearly every disease, favorable conditions means fish in poor health with the most frequent reason being poor water quality.


It is another parasite that behaves much like ick, appearing as stains 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.

Remedy: medication for parasites

They’re both parasites using a similar life cycle- part of which is attached to a fish and part of which is spent free swimming. Killing them is more or less impossible while they are safely burrowed under the fish’s skin. It is only when they emerge into the open water to search for a new host that they are vulnerable. This means treatment can take a while. Medication has 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 need a fish host to complete their life cycle. So, should you move all your fish to quarantine for treatment, any parasites left at the main tank will die off after about a month.


Signals – fish is bloated, possibly with the scales protruding giving it a pine cone look

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

Treatment: medicine for bacterial infections, aquarium salt

Unfortunately by the time the signs are clearly visible it is often too late to save the fish. Still, the best strategy is to move the fish to quarantine and begin administering an antibiotic, preferably in the shape of an medicated food. Adding a little bit of aquarium salt may also help the fish expel some of the extra fluid thereby relieving the swelling. Use around one tablespoon per five gallons.

Swim Bladder

Signals – 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, just with air rather than water. They use it to keep themselves upright and in the appropriate depth. When it becomes damaged or otherwise perturbed the fish is no longer able to restrain this air and so will usually either sink to the bottom or float to the surface, often in an off-kilter orientation.

Treatment: fasting/cooked peas, medicine for bacterial diseases

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

Pop Eye

Signals – fish has a protruding eye

A pretty self explanatory title, the fish’s eye or eyes bulge out of the fish’s head as if they’re on the verge of falling out entirely. It’s basically an inflammation of the eye causing it to swell and protrude. Once again this isn’t so much connected to any one specific cause but rather might originate from a few possible sources, namely injury or an illness.

Injury due to fighting or possibly from bumping into something is probably the most common. A good indicator that this is a the cause is if just one eye is affected. In cases like this the best you can do is stop any fighting and provide your fish with a calm 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 assessed for signs of injury or fighting. A good medication is the best course of action here.

Add 1 tablespoon per five gallons and watch to see if it has any impact. Bear in mind this can help alleviate 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.

Signals – fish has a cloudy eye

The creatures of the world have all sorts of interesting eye with various colours and shapes and whatnot. 1 thing they all have in common, though, is that the center is clear and nice. If you ever notice your fish’s eye or eyes getting milky then something is not quite perfect.

treatment: improve conditions, medication for bacterial infections

There are a handful motives a fish’s eye may become cloudy. Parasites or germs are possible causes, particularly if the eye has been injured. Poor diet or even cataracts because of old age are possible culprits as well. But, the most probable cause falls based on so many other problems- poor water quality. Getting the tank is fantastic shape should be your first concern if your fish develops muddy eyes. Pay particular attention to the pH as a particularly low pH is considered to contribute to this problem. Check that you’re feeding your fish an appropriate diet also. With better water quality and a suitable diet the problem should clear in a few weeks. However, if the problem does not clear after a few weeks with enhanced conditions an antibiotic can be attempted.

Hole in the Head/Lateral Line Disease

Signs- fish has holes in its head

The title is fairly self explanatory, and the indicators are easy to spot, so all that leaves is the cause. Why are there undesirable holes in your fish’s head? Unfortunately there is 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.

Remedy: improve conditions, diet change

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

Signals – the fish’s fins are deteriorating

Fin rot is another ailment that’s not so much about a particular contagion as opposed to a consequence of the fish’s overall health. Fin rot typically only affects fish which are already stressed or weak because of something else such as poor water quality, malnutrition, bullying, or perhaps some other illness, at which point bacteria move in and begin feasting on your bad fish’s fins.

treatment: improve conditions, medication for bacterial infections

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

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

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