Marine Fish Diseases- Signs, Causes, And Cures

The world of disease is persistent if nothing else. For every life form out there, there’s another one looking to get ahead by ruining the first one’s day. Your fish are no exception. And, much like with the various illnesses that can befall humans, there are a ton of different afflictions marine fish can encounter. This guide doesn’t cover every last one but instead focuses on spotting and curing the handful of diseases you are most likely to encounter.

First, a few general notes on how to approach illnesses:


Spotting Illness
When an illness breaks out catching it early is everything. Some diseases move extremely quickly or don’t become obvious until it’s nearly too late. By and large illnesses will manifest visually, either as an odd behavior or as a change in the appearance of your fish. Hopefully you already have a decent idea of what qualifies as normal appearance and behavior for your particular fish. Here are some things to look out for:

-paleness or color change (Note that many marine fish have some limited ability to alter their color, particularly at night.)
-heavy respiration
-loss of appetite
-hiding or isolation
-loss of equilibrium
-scraping or rubbing on rocks and/or other objects
-clamped fins (the fish holds its fins tightly against the body)
…as well as the more obvious signs such as open sores, swelling, and so on.


Water Quality Is Everything
Many of the illnesses you’re likely to run into tend to only grab hold in fish that are already in less than perfect health. And what is the #1 cause of poor health in fish? Poor water quality. Whenever any potential problem arises it should always be the first thing you check. Especially so in the marine environment, even tiny irregularities in the water’s parameters can have a big impact on your fishes’ health. Ammonia, nitrate, and pH are the big ones to look at right away, as are temperature and salinity. Also remember that making sure they are stable is equally important so check for things like big swings throughout the day. And, even if everything checks out, a water change is still a good first move. High water quality is the universal first step towards recovery from disease.


Dosing Medications and the Quarantine Tank
Medications, by and large, fall into the category of ‘still better than dying’, basically meaning the benefits outweigh the side effects. Many of them can also be quite harmful, if not fatal, to the other inhabitants of your tank besides the fish. Properly dosing can be tricky in the main tank as well and water chemistry can be affected greatly. This is why moving any sick fish to a quarantine tank for treatment is so important. Adding medication to even a fish only tank, let alone a reef tank, can be disastrous. Some medications do claim to be reef safe, but even then it’s a chance most aren’t willing to take. Don’t risk your main aquarium. Setting up a quarantine tank is easy and doesn’t take much- you may even have everything you need already.


Copper Based Medications
Copper based medications and nasty stuff but unfortunately one of the only effective treatments known for many parasites. They are hard on fish and thus must be dosed exactly and very carefully. Copper is also extremely toxic to most marine invertebrates- corals, crabs, snails, and so on. But the real problem is this- it’s hard to get rid of copper once you use it. It will contaminate everything in a system where it is used, including the aquarium itself, such that none of the equipment should be used for invertebrates again without great risk. If you find yourself battling ich or something similar and need to treat with a copper based medication make sure you only do so in a quarantine tank and make sure you keep every tool and piece of equipment clear of your main aquarium.


Carbon and Medications
Carbon’s job in filtration is to suck up chemicals. This is a good thing if you’re trying to take the water from merely clean to sparkling. But it’s not so good when you consider that it’s more than happy to suck up most medications as well. Should you have the need to use any type of medication remember to not use any carbon in the filter to avoid this issue.

And on with the diseases!


Marine Ich
-small white spots scattered around the fish’s body and fins
-fish rubbing against rocks or other objects in the tank

Ich is a very common and recognizable parasite that a lot of aquarists encounter eventually. The marine version is a separate species from its freshwater counterpart but it behaves in a pretty much identical fashion. The parasite has a fairly complicated life cycle, part of which is spent attached to your fish and part of which is spent free swimming. Unfortunately they’re only vulnerable during their free swimming stages and so treatment can take some time.


Marine Velvet
-fine white spots that give the skin an almost velvety appearance
-fish rubbing against rocks or other objects in the tank
-heavy breathing
-erratic behavior

Velvet is another parasite with a life cycle and behavior similar to ich. It tends to start in the gills and spread from there. Unfortunately this means by the time its characteristic velvety appearance is visible the infection has likely progressed quite far. Much like ich, the parasite is only susceptible to treatment during its free swimming stages and so eradication can take a while.


Black Spot
-small black spots scattered around the fish’s body and fins
-fish rubbing against rocks or other objects in the tank

Often associated with yellow tangs, black spot can actually infect most any fish. The cause is a small darkly colored parasitic worm which latches onto the body of its victim. While it’s not as deadly as other parasites it is still something to be concerned about- a parasite is a parasite. Again like ich part of the worm’s life cycle is spent independent of the host fish. However, unlike ich they can survive for quite some time on their own making total eradication more difficult.


When it comes to cures, all three of these parasites are fairly similar. A freshwater bath can remove some of the parasites and can be a good option to bring some quick relief to heavily infested fish. However, to eradicate the parasite, a copper based medication is the most effective method. Remember, if you have any invertebrates or ever plan to add any to your tank you’re best off doing any medicating in a separate tank as the copper will kill them and can linger for a long time. Finally, even if only some of your fish are affected, moving all of them to quarantine for treatment is preferred. They likely all harbor at least a few and completely emptying the main tank of fish for a few months (unfortunately it can take quite some time) will starve out any free swimmers that get left behind, preventing another outbreak after treatment.


-cloudy eyes
-rapid breathing
-fish rubbing against rocks or other objects in the tank
-faded or off-color patches
-loss of appetite
-frayed fins

Fluke is a generic term for a whole range of parasitic flatworms that can attack your fish. The worms are larger than other parasites you may encounter, about 1 millimeter in size, but still quite difficult to spot, particularly on a moving fish. They also tend to hide under scales or in the gills which makes it even harder. Typically, outbreaks happen with the addition of a new fish that brings the parasites with him. If you suspect any new fish you purchase have flukes they need to be treated immediately. Flukes are a dangerous parasite that can quickly spread and take down even healthy fish.

Like other parasites, a freshwater bath can dislodge the bulk of them and bring some quick relief to heavily infected fish. It can also help in positively identifying the infection as flukes if you aren’t certain- the dead flukes left in the freshwater bath will be easy to spot. But, like most parasites, this is not an effective long-term cure. To eradicate the flukes the fish will need to be quarantined and treated with an appropriate anti-parasitic medication.


-heavy or labored breathing
-fish rubbing against rocks or other objects in the tank
-heavy production of excess mucus

Brooklynella is a nasty parasite capable of killing fish very quickly. While it’s commonly known as clownfish disease, Brooklynella plays no favorites when it comes to which fish it will infect. The parasite focuses primarily on the skin and gills, destroying them. The telltale sign of this disease is the excess body mucus, which will be easily visible as it builds up and sheds from the fish’s body. Secondary bacterial infections are quite common as well, exacerbating the problem. Unfortunately by the time the fish is visibly infected their health is likely quite poor, making the chances of survival lackluster.

Formalin is the key. This a formaldehyde based medication and as you can imagine is quite nasty stuff. Never add formalin directly to the tank itself as it will kill pretty much everything. Treatment is instead administered in a series of short dips in a separate container filled with medicated tank water. Heavy aeration is necessary in the treatment vessel to keep the oxygen levels up. The dips should last no more than an hour and the fish should be monitored at all times for signs of any distress. Should any occur the fish should be removed immediately. Repeat the baths everyday for about a week. A medication for bacterial infections should be added to the quarantine tank to curb secondary infections as well.


-irregular lumps resembling cauliflower on fish’s fins and body

Lymphocyctis is one of the few viral infections you may come across. The most obvious sign a fish is infected is white, lumpy, cauliflower-like growths on the fish’s body, hence the common name “cauliflower disease”. The virus is quite infectious and so can spread rapidly throughout a tank’s population.

This is one of those times when the situation looks a lot worse than it actually is. Lymphocyctis is rarely fatal and with good conditions will often clear up on it’s own in time. Furthermore, as yet nothing has been found to effectively treat the virus. The only thing to watch out for is the possibility of secondary bacterial infections.


-open sores
-fraying or bloodshot fins
-cloudy eyes
-loss of appetite
-weight loss

Vibriosis is a bacterial infection that most commonly manifests as open sores, although lots of other symptoms are possible (cloudy eyes, swelling, weight loss, etc). Basically it’s an aggressive bacteria that ravages your fish’s health. It tends to only strike when a fish is already in poor health as healthy fish can easily fend off the infection.

The first step is to figure out why the fish was in poor health in the first place. Water quality is a prime culprit, but there are other possibilities as well (overcrowding, bullying, poor diet). Simply correcting the issue is often enough to turn the fish’s health around. Beyond that, quarantine and an antibiotic are your best bet.

Also note that it is transmissible to humans meaning extra caution should be used when handling any infected fish.


-fraying or bloodshot fins
-cloudy eyes
-loss of appetite
-weight loss
-scale loss
-color loss

Commonly known as wasting disease, this illness causes the affected fish to do just that- waste away. Lesions, frayed fins, and weight loss are common signs. Positive identification is very tricky since it can manifest in so many ways and may not show noticeably at all.

Unfortunately there’s not much that can be done. The bacteria that causes it is extremely resilient and no effective treatment has been found. The best you can do is remove the fish from the system before it has a chance to spread.

This is another disease that is transmissible to humans. Should you suspect a fish is infected or has died from this disease use extra caution in the manner in which you proceed. Avoid making any contact with the infected fish, living or dead.


Marine Fungus
-fraying fins
-loss of color
-open sores
-roughened skin

Fungal infections are fairly common in freshwater aquariums and some marine diseases can take on a similar appearance. However, actual fungal infections are quite rare in the marine environment. If you encounter something that reminds you of freshwater fungus it is likely something else. Nonetheless they can happen and like many other illnesses tend to grab hold in fish that are already weakened. Unfortunately they are much harder to spot as they primarily affect the internal organs. It can manifest as discolored or roughened skin, sores, loss of appetite, and a range or other generic symptoms, making a positive diagnosis difficult.

Because of the rarity and difficulty in identifying fungal infections not much effort has been put into finding an effective treatment. Some anti-fungal medications are available which may work. Otherwise, the best you can do is improve the tank’s conditions as much as possible.


Fin Rot
-deteriorating fins

Fin rot, while caused by bacteria, is more a sign of a fish in bad general health. Much like with people, when their health is poor it opens the door for all sorts of infections. This can begin as fraying or a bit of redness and can eventually consume the entirety of the fish’s fins.

Improving the fish’s conditions is the #1 goal. An antibiotic can help if the deterioration has progressed to an extreme level, but often just getting the fish back in good health will be enough to turn this around.


-bloating or swelling of the body
-raised scales

Dropsy, or ‘pine coning’ as it is sometimes known (as the raised scales and swollen shape can make a fish look like a pine cone), is a sign of an internal bacterial infection. The infection is causing fluid retention causing the fish to puff up.

This is another one that’s more of a symptom than a specific illness. The affected fish should be treated with an antibacterial medication and given the best conditions possible, naturally. You may also try checking that the magnesium level is high enough as some suspect it to play a role.


Hole in the Head/Lateral Line Disease (HLLE)
-pits forming in the head and along the lateral line
-loss of color

A pretty self explanatory name- the fish develops holes in its head (usually located roughly around the eye) and down the length of its body along the lateral line. As the disease advances the fish’s color can begin to fade.

While it isn’t known definitely what causes HLLE the leading theory points to nutrition deficiencies. Change your fish’s diet to include a wider variety of quality foods, particularly those enriched with extra vitamins and minerals.


Environmental Issues
Some of the most common health problems aquarium fish face actually stem not from disease or infection but from substandard living conditions, usually poor water quality. Of course once a fish is in poor general health it’s often not long before a true illness moves takes hold. Even so the first step in treating these problems should be getting the water back in good condition. This alone can frequently turn your problems around.

Here are some potential issues your water may have:

Ammonia/Nitrite: It’s the biological component of your filtration that keeps these in check. If it becomes damaged somehow, or was never properly established, these two can build up and cause problems. If you ever have very sudden deaths or see your fish at the surface gasping for air then this is something you should test immediately.

Solution: Get the biological filtration working. If it was never properly established then you’ll be in for a rough ride while cycling the tank. Things will be easier if it’s just a hiccup in the system, but either way lots of water changes are in order to bring the levels down.

Nitrate: Nitrate is the end product of biological filtration. Without regular water changes or some other method of export it can build to dangerous levels over time.

Solution: With regular water changes nitrate should never be a big issue.

Copper and Other Heavy Metals: Copper is highly toxic to invertebrates and at high enough levels hazardous to fish as well (some more so than others). Unfortunately many medications happen to be copper based. This can make treatment of fish tricky and opens the potential for contamination of the main tank.

Solution: Test kits are available if you suspect copper may have gotten into your tank. To get rid of it water changes and special filter media that will absorb it are the way to go. Similar to nitrate, this isn’t something that’s likely to be an issue without a serious mistake occurring. Make sure you never use any copper based medications in your main tank nor ever add any object with exposed copper. Keep separate tools and equipment for any hospital tank to avoid cross contamination.

Other Contaminates: Soaps, perfumes, and all manner of other household chemicals are all potential contaminates with the potential to seriously compromise your tank.

Solution: These are the hardest to identify. About the only way to even potentially identify that something got into the tank that shouldn’t have is by diligently eliminating all other possible causes of an issue. The best you can usually do when it seems something other than the usual suspects made its way into your tank is to start in with the water changes.

Source Water Issues: If you don’t use a reverse osmosis unit to filter your water before adding it to your tank there is the potential for all sorts of other things to make it through. Some potential contaminates include nitrate, phosphate, silicate as well as copper and lead which can leech from water pipes. Even if the concentration is tiny over time as water is continually added to the tank they can build up.

Solution: Either find a cleaner source of water or bite the bullet and get a water filter. Distilled or some other form of purified water may be available for purchase locally, but if you have more than a desktop sized aquarium that can get expensive quickly. A reverse osmosis unit is expensive up front but well worth it in the long run.

Low Oxygen/Poor Gas Exchange: Fish, corals, and pretty much everything good living in your tank need to breath. Without proper aeration this can become an issue. In fact aeration not only puts oxygen into the water but also drives off other gasses, namely carbon dioxide. This is rarely an issue in marine tanks but can pop up under the right circumstances. A good indicator that this may be an issue is the pH dropping during the night. The problem is exacerbated during the night when photosynthesis is no longer contributing to the oxygen level.

Solution: Pretty simple- increase aeration. Aim for enough water movement that the surface is covered in ripples. At least partially uncovering the tank helps as well so fresh air can actually get to the water.


Remember, always check the water first whenever a health issue arises. Keeping the water quality in tiptop shape is key to recovery from as well as prevention of disease. Use of a quarantine tank is important too. Proper identification of any illness is absolutely necessary before moving on to more drastic cures, particularly medications.

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