Fat Lip Fish Issues – How To Identify When There’s a Problem

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If you’ve noticed that your fish has swollen lips, there are a few possible causes – some of them quite benign and others very serious.

‘Fat Lips’ or ‘Duck Lips’ can be caused by fish genetics, mild injuries, viruses, or bacterial infections.

In the latter case, fat lips can indicate a very serious illness and may cause death in your fish in as little as a few days.

To understand whether your fish’s symptoms need treating, you’ll need to know a few more details about these conditions. Let’s investigate together.

Fat Lip Fish Species and Genetics

Some varieties of fish just naturally have big lips. In freshwater aquariums African Cichlids, Pacus, Kissing Gouramis, and fish belonging to the carp family often have larger lips than most species.

In saltwater aquariums, you’re likely to find big, puckered lips on Snappers, Groupers, Wrasse, and especially the forever-pouting Triggerfish!

Some species of fish have mouths that look more like suckers. Plecos and Chinese Algae Eaters are famous for their big sucker mouths that they use to graze algae, effectively cleaning your aquarium.

Other times, individual fish may have a slight genetic mutation that gives them especially big lips, without it causing any significant health problems during their lives.

Fat Lips Caused By Injuries

Aquarium fish can be quite skittish at times.

This is hardly surprising, given that they’re naturally adapted to live in large expanses of water rather than inside four glass walls, and they can easily damage themselves as they become over-excited or nervous and swim into the glass.

Fish that have swum fast into aquarium glass may develop bumps and bruises on their lips that will usually reduce in size within a day or two.

If symptoms persist, take a closer look to make sure that a bacterial infection isn’t present.

Fat Lips Caused By A Virus

Fish viruses can make different parts of their bodies swell up, including the lips.

One of the more common examples of enlarged lips caused by a virus is the Lymphocystis Virus Disease (LCDV) which can affect the fish’s fins, skin, and gills, but also sometimes lips too.

Lymphocystis disease causes wart-like nodules to develop on fish, which can spread to different areas of the body.

Although the disease is not treatable, fortunately, Lymphocystis is rarely fatal, and with good husbandry, clean water, and a diverse, nutritious diet, your fish should make a full recovery in just a few weeks.

Fat Lips Caused by Bacterial Infections (Potentially Fatal)

If fat lips or ‘duck lips’ develop suddenly in your fish, it could well be a bacterial infection – which could be very serious.

Bacterial diseases such as Columnaris (sometimes referred to as cottonmouth) and Aeromonas must be treated very promptly to avoid disaster.

Cottonmouth is a contagious disease that results from an infection caused by the bacterium Flavobacterium columnare.

It’s more common in aquariums that have high bioloads, low oxygen levels, and generally poor water quality. It can only be effectively treated with antibiotics, and if left untreated will usually kill fish within 72 hours.

Other bacterial infections that cause ulcers such as Aeromonas can also be very serious and need to be treated swiftly with antibiotics such as Terramycin®, or Remet-30®.

There are debates within the aquarium community about whether to administer antibiotics via fish food or in the water.

Giving antibiotics in fish food is a direct way to get the medicine into the fish’s internal organs without the need to treat the entire tank or stress the fish with a bath. On the other hand, if the fish is very sick and refuses to eat, administering antibiotics orally can become troublesome.

For all but the most experienced aquarists, you’ll need to consult a trained veterinarian instead of treating these serious diseases by yourself.

How To Avoid Mouth Infections in Fish

fat lip fish

As you can see, bacterial mouth infections in fish can be extremely serious and are much better avoided than treated.

Here are a few tips for how to avoid them, or how to provide the right conditions for your fish to recover from infections if they’re already being treated:

Use a Good Filter

Good aquarium filters are an essential part of any tank setup. Although people sometimes try managing aquariums or fish bowls without a filter, there’s no longer any logical sense in doing so.

Filters clean, clear and oxygenate your water, effectively removing toxins like ammonia and nitrites 24 hours a day.

They’re also increasingly affordable, with the most basic setups costing less than $20, and will save you a lot of extra cleaning hours that you’d need to do without one.

Make Regular Partial Water Changes

Even with a good filter, you’ll still need to change a portion of the water regularly to remove substances that the filter can’t remove alone and to replenish the mineral content of the tank.

Water changes of between 15 – 35% are recommended every 1 – 2 weeks, depending on the stocking density and type of fish.

Give Your Fish a Varied, Healthy Diet

As well as clean water, your fish need to be well nourished to maintain a healthy, robust immune system.

The extra minerals and proteins found in meaty foods such as bloodworms, daphnia, and brine shrimp can make a big difference to your fish’s overall health and well-being.

Keep a Close Eye on Your Fish

The best overall piece of advice for avoiding problems in a fish tank is to spend lots of time with your fish.

When you’re watching them attentively every day, your natural concern and regard for your fish will increase, and you’re much more likely to spot any issues before they become a menace.

Watching your fish is also a healthy activity for you! Scientific research has indicated that spending time watching fish has the potential to brighten mood, reduce pain, and even improve nutritional intake in humans.

So you need never feel that time spent with your fish is time lost, in fact, you can rest in the knowledge that it’s doing a great deal of good for you too.


‘Fat Lips’ or ‘Duck Lips’ in fish have several causes, with most being fairly harmless.

If your fish has developed sudden symptoms, however, it may indicate a bacterial infection that needs immediate treatment to avoid the death of your fish.

Bacterial mouth infections are much more easily prevented than cured, so always be sure to maintain a clean aquarium as well as a diverse feeding regime, and spend lots of time with your fish to ensure they remain healthy and well for many years to come.

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