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Why Is My Pleco’s Mouth Wide Open? (5 Easy Solutions)

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Plecos are resilient fish, making them popular with many fish lovers. Their stunning appearance and fascinating habits are hard to ignore.

However, they do come with their set of issues. I once noticed one of my Plecos frequently opening its mouth, showing signs of distress that really concerned me.

Luckily, after careful research, I discovered a fix for the problem. Now, I’m ready to guide you on how to tackle such situations. Let’s get started.

Is Having Their Mouth Wide Open in Plecos Normal?

No, having their mouth wide open constantly is not normal for Plecos. This behavior might indicate stress or health issues in the fish.

  • Breathing Patterns: Plecos breathe through their gills, not by opening their mouths. A wide-open mouth could indicate difficulty in respiration.
  • Natural Behavior: Plecos use their mouths mainly for feeding, especially algae from surfaces. They don’t keep them open without reason.
  • Stress Indicators: Like many fish, Plecos show physical signs when stressed. An open mouth might be a response to poor water quality or aggression from tankmates.
  • Health Concerns: If a Pleco is keeping its mouth open constantly, it might have an internal or external infection affecting its ability to close its mouth properly.

Also Read: Stress In Pleco Fish

Why Is My Pleco Keeping Its Mouth Wide Open?

A couple of reasons might be forcing your Pleco to open its mouth, with the most common being:

1. Oxygen Deficiency

If your Pleco is keeping its mouth open persistently, it might be experiencing a lack of oxygen in the water. This forces the fish to try to take in more oxygen through its mouth.

  • Gill Movement: Plecos primarily obtain oxygen through their gills. If gill movement becomes rapid, it suggests the Pleco is working harder to get enough oxygen.
  • Surface Gasping: In severe oxygen deficiency, Plecos might be seen near the water’s surface, opening their mouths wider as an attempt to capture more oxygen.
  • Tank Observations: Overstocked tanks or those with poor water circulation can lead to diminished oxygen levels, impacting the Plecos’ respiratory comfort.

2. Attempt to Regulate Buoyancy

Just like some fish gulp air to regulate their buoyancy, a Pleco might keep its mouth open if it’s struggling to maintain its position in the water column.

  • Swim Bladder Function: The swim bladder in Plecos helps them stay buoyant. If compromised, they might resort to other methods like mouth opening to stabilize.
  • Dietary Concerns: Overfeeding or feeding foods that cause gas can impact the swim bladder’s function, leading Plecos to adopt atypical behaviors.
  • Erratic Swimming: If your Pleco is displaying unusual swimming patterns alongside the open mouth, it’s a strong sign of buoyancy issues.

Also Read: Why Is My Pleco Swimming Erratically?

3. Water Quality Issues

Poor water quality can cause stress and health problems for Plecos, leading them to display signs of discomfort, like keeping their mouths open.

  • Toxic Parameters: Elevated levels of ammonia, nitrites, or nitrates can stress Plecos. Regular water testing ensures these toxins stay at safe levels.
  • Visible Signs: Aside from the open mouth, Plecos might show other signs like faded colors or clamped fins if water quality is poor.
  • Frequent Changes: Abrupt or frequent changes in water parameters (like temperature or pH) can stress Plecos, making them more susceptible to displaying signs of unease.

4. Disease or Mouth Injury

Plecos might sustain injuries or contract diseases that specifically affect their mouth, causing them to keep it open.

  • Physical Trauma: Plecos, with their penchant for scouring surfaces, might accidentally injure their mouths on sharp tank decorations or substrates.
  • Bacterial or Fungal Infections: If a Pleco’s mouth seems swollen or discolored, it could be suffering from an infection, making it harder to close.
  • Feeding Observations: A Pleco with a mouth injury or disease might show reluctance or difficulty while feeding. Watch closely during feeding times for any abnormalities.

5. Territorial or Mating Display

Plecos can be territorial, and males especially might display certain behaviors, including opening their mouths wide, as a show of dominance or during mating rituals.

  • Tankmate Interactions: Observe how your Pleco interacts with other tankmates. An open mouth can be a warning sign to other fish to keep their distance.
  • Breeding Season Indicators: During breeding seasons, male Plecos might open their mouths more frequently as a display towards potential mates or rivals.
  • Habitat Observations: Ensure there are enough hiding spots in the tank. Lack of sufficient spaces can intensify territorial behaviors, with Plecos resorting to more aggressive displays like open-mouth threats.

Also Read: Why Is My Pleco Hiding?

How to Treat Plecos That Consistently Keep Their Mouths Open

Here’s what you can do if your Pleco keeps its mouth open:

1. Increasing Oxygen Levels

A deficiency in oxygen can lead to Plecos keeping their mouths open. It’s essential to enhance tank oxygenation to ensure the well-being of your Pleco.

  • Air Stones and Bubblers: Installing an air stone or a bubbler can increase water oxygenation. My recommendation: Hygger Aquarium Air Stone (link to Amazon).
  • Water Movement: Use filters or powerheads to ensure water circulates efficiently. For example, a 600-800 GPH (Gallons Per Hour) filter is good for a 75-gallon tank.
  • Live Plants: Aquatic plants, like Anubias or Java Fern, can improve oxygen levels. Planting 5-6 stems per 20 gallons can have noticeable effects.
  • Regular Water Changes: Replace 20-25% of tank water weekly. Fresh water introduces more oxygen and dilutes harmful substances.

Also Read: Do Plecos Need A Bubbler?

2. Adjusting Tank Environment

The habitat and tank setup play a pivotal role in Pleco behavior. A suitable environment can prevent the issue of Plecos keeping their mouths open.

  • Smooth Decor: Ensure decorations lack sharp edges. For instance, using rounded river rocks can reduce the risk of mouth injuries.
  • Adequate Shelter: Providing caves or driftwood can offer Plecos hiding spots. One large cave or two smaller ones per Pleco should suffice. My Plecos absolutely love this Jabukosu Aquarium Cave (link to Amazon).
  • Tank Size Matters: For common Plecos, a minimum of a 75-gallon tank is recommended. Overcrowding can lead to stress and abnormal behavior.
  • Temperature Stability: Maintain a consistent temperature between 74-80°F using heaters and monitoring with a reliable thermometer.

Also Read: Pleco Fish Tank Setup

3. Improving Water Quality

Maintaining optimal water conditions is paramount for the health of your Pleco. Poor water quality can result in the unusual behavior of keeping their mouths open.

  • Regular Testing: Using a water testing kit, check for ammonia, nitrites, and nitrates bi-weekly. Ideal readings: 0 ppm ammonia, 0 ppm nitrites, <20 ppm nitrates.
  • Natural Filtration: Live plants can help absorb toxins. Plants like Egeria Densa can consume excess nitrates, improving water clarity.
  • Frequent Water Changes: As mentioned, change 20-25% of the water weekly. This practice dilutes toxins and introduces beneficial minerals.
  • Use Water Conditioners: Adding water conditioners during changes neutralizes harmful chlorine and chloramines. I personally use the Tetra AquaSafe (link to Amazon).–roMpADL9Q?si=1Uvl-SKBc9MqO_wO

4. Treating Disease or Injury

If a disease or mouth injury is causing your Pleco to keep its mouth open, prompt treatment is necessary to ensure its health and comfort.

  • Isolate Affected Fish: Use a hospital or quarantine tank. For a single Pleco, a 20-30 gallon tank with a sponge filter is a good start.
  • Consult a Veterinarian: For suspected diseases or severe injuries, seek advice from an aquatic vet for precise diagnosis and treatment options.
  • Over-the-counter Treatments: Depending on the ailment, medications like Melafix or Pimafix can be beneficial. Always follow the dosage recommendations on the label.
  • Maintain Good Nutrition: Offer a balanced diet, including algae wafers and occasional fresh vegetables, to bolster your Pleco’s immune system. My recommendation: Invert Aquatics Mini Algae Discs (link to Amazon).

Also Read: Pleco Fish Diseases

5. Observing and Adjusting Social Dynamics

Interactions with other tank inhabitants can affect Pleco behavior. Adjusting the social environment can address issues related to territorial disputes or mating rituals.

  • Watch for Bullies: Observe your tank during feeding times or when lights first turn on. If other fish are chasing or nipping at your Pleco, consider rearranging tankmates.
  • Increase Hiding Spots: More caves or driftwood can reduce territorial disputes. For a tank with multiple Plecos, provide several hideouts to reduce competition.
  • Tankmate Compatibility: Some fish species might not be ideal companions for Plecos. Research and choose tankmates that have similar temperament and environmental needs.
  • Breeding Considerations: If mating displays become aggressive, consider separating males or adjusting the male-to-female ratio to reduce conflicts. Typically, a 1:2 or 1:3 (male to female) ratio can help.

Also Read: Pleco Fish Tank Mates

Will a Hungry Pleco Keep Its Mouth Open?

No, a hungry Pleco does not typically keep its mouth open as a direct sign of hunger. However, hunger may manifest in other behavioral or appearance changes in the Pleco.

  • Increased Activity: Hungry Plecos might scour the tank more actively, searching for food on surfaces, substrates, or even the tank’s glass.
  • Thinner Appearance: If Plecos don’t receive enough nutrition, they may start to look noticeably thinner or underfed over time.
  • Nipping Behavior: While Plecos are generally peaceful, extreme hunger might lead them to nip at plants or even other tankmates’ fins.
  • Visible Sucker: A very hungry Pleco may frequently stick its sucker mouth to the tank glass, appearing as if it’s trying to feed on any available algae or biofilm.

Do Pleco Species Differ in Mouth Behavior?

Yes, different Pleco species can exhibit different mouth behaviors based on their specific habitats, diets, and lifestyles.

While many generalities exist, certain species nuances affect their mouth behavior.

  • Common Pleco (Hypostomus plecostomus): This species uses its mouth primarily for grazing on algae-covered surfaces, often sticking to the tank’s sides or bottom.
  • Bristlenose Pleco (Ancistrus spp.): Recognized by tentacle-like growths around their mouths, they are adept at foraging on wood and tend to “suck” more frequently.
  • Zebra Pleco (Hypancistrus zebra): A carnivorous species, their mouth behavior is more geared towards sifting sand for tiny invertebrates than scraping algae.
  • Rubber Lip Pleco (Chaetostoma spp.): Their mouths are structured to graze on smoother stones in fast-flowing streams, making them less prone to “sucking” behaviors in calmer tank waters.
  • Clown Pleco (Panaqolus maccus): Preferring driftwood as a primary food source, their mouth behavior involves clinging and gnawing on wood, making it distinct from algae-grazing Plecos.


For quick readers, here’s a short recap:

  • A Pleco keeping its mouth wide open continuously is abnormal and could indicate stress, respiratory issues, or other health concerns.
  • Common reasons for this behavior include oxygen deficiency, buoyancy regulation struggles, poor water quality, mouth injuries, or territorial/mating displays.
  • To address a Pleco with an open mouth, one should enhance tank oxygenation, adjust tank environment, improve water quality, treat any diseases or injuries, and observe and adjust social tank dynamics.
  • Hunger does not directly result in a Pleco keeping its mouth open. However, signs of hunger in Plecos might manifest as increased activity, a thinner appearance, or increased use of their sucker mouth.
  • Different Pleco species exhibit varied mouth behaviors based on their specific habitats, diets, and lifestyles, from algae grazing to wood gnawing or sifting sand for food.