Guppy Fish Mouth Open – Reasons and Solutions

Guppies are energetic, hardy, and widespread fish in the aquarium business. They’re some of the most prolific breeders in captivity and display a mind-blowing range of colors and patterns.

They are also peaceful and joyful and can adapt to any community setup, provided their tank mates are equally friendly.

But the fish will sometimes experience health issues that are often detectable via the guppy’s behavior. One of these behaviors refers to the guppy keeping its mouth open longer than normal.

Reasons Guppy Has Open Mouth

It’s not normal for your guppy to keep its mouth open for too long at a time. If that happens, there are some potential culprits at play, including:

– Food Stuck

Believe it or not, it’s actually common for fish to choke on food occasionally. You may not notice it because the situation usually resolves on its own, but it does happen. Generally, it won’t affect the fish too much, but there are times when it could get dangerous.

Guppies can experience food getting stuck in their throats for several reasons:

  • Too large – Pellets and flakes are generally to blame for this. If the food particles are too large, the fish may choke on them. In some cases, the guppy won’t even be able to eat them. This is a critical point since not all guppies are of the same size. Smaller ones need to eat smaller food, especially when they’re younger and have tiny mouths.
  • Starving the fish – If your fish doesn’t have a balanced feeding pattern, it may starve in between the meals. This could cause it to rampage through its food when it finally arrives. As a result, the choking hazard is a lot higher.
  • High food competition – Guppies are generally peaceful creatures but will fight for what’s theirs. This includes the food. If you have too many guppies and not enough food, you best believe they will fight for it. This will lead them to eat in a hurry, trying to outcompete other guppies. The situation is even worse when the food is insufficient, or the meals are rare in between.

The solutions:

  • Break down the food – If the pellets, flakes, or live food are too large for your guppies, break them into smaller pieces. This is especially important for guppy fry due to their small mouths and the tendency to eat fast.
  • Ensure a stable feeding pattern – Adult guppies need 2-3 meals per day to remain full. Their diet has to be diverse to ensure optimal nutrient intake, and the fish should receive enough food to keep them satiated. Guppy fry require meals higher in protein and more frequent feeding, especially during their first few weeks of life. Consider feeding them 3-4 times per day in small portions. A stable and consistent feeding pattern should keep your guppies safe from starving and overeating once the food arrives.
  • Minimize the impact of food competition – It’s in guppy’s blood to compete over food, mating rights, and territory with other guppies. Your job should be to lower these tendencies to ensure healthier dynamics between the fish. If you have a larger guppy population, spread out the food over the entire water’s surface. This will make sure every guppy gets to eat properly.

Regarding the last point, always monitor your guppies’ interactions. Some can have a bullying mentality, be over-dominant, and display unusual aggression towards other guppies. This generally happens among males. If this is an issue, try to mitigate the fish’s aggression, even if that means removing the aggressor from the environment.

It’s better than having several fish starving over one violent bully.

– High Stress

Fish stress is quite common in tank fish. The main reason is the fact that they’re trapped in a close environment, so they will have to face various stressful situations. They have nowhere to run. Some of these circumstances include:

  • Aggressive tankmates – Guppies are peaceful and docile and enjoy equally friendly tank mates. Pairing them with predators, larger fish, fin nippers, or overly playful and energetic species is suboptimal. Guppies will experience stress, and if they have nowhere to hide, they will become sick. Prolonged stress is known to affect the fish’ immune system, making them prone to parasites and infections.
  • Poor water conditions – Guppies require clean, fresh, and well-oxygenated waters, free of any dangerous chemicals like ammonia, nitrites, chlorine, or heavy metals. Any of these substances can affect their health and behavior. The guppy’s tank requires frequent cleaning and maintenance to prevent these problems long-term.
  • Inadequate food or feeding – Overfeeding guppies will lead to constipation and other digestive problems. Underfeeding them may increase their irritability and affect their interactions. Providing guppies with improper food of doubtful quality can also affect their health. All these issues will stress the fish, and it will show.
  • Overstocking – Guppies require sufficient space to swim and live comfortably in their environment. Each guppy needs around 2 gallons of water, so a group of 5-6 requires at least 20 gallons of space. This should also include the room necessary for the tank equipment, plants, and other decorative elements.
  • Diseases and infections – Guppies are generally hardy, but they’re not impervious to disorders. Parasitic and bacterial infections can occur at times, especially due to poor water conditions. Guppies will display a variety of symptoms, stress-related behavior being one of them.

The solutions:

The solutions to all these issues are rather self-explanatory:

  • Ensure enough space – Invest in a larger tank to make sure your guppies have sufficient space available. A more open environment will keep them active and prevent overcrowding in the long run. You should also control your guppies’ breeding for the same reasons. Guppies are highly proficient breeders, so they can multiply fast. Keep their breeding under control, and you should be fine.
  • Ensure optimal water conditions – Tank maintenance, a good filtration system, and regular water changes. These 3 strategies are necessary to keeping your guppies’ water clean and fresh. I suggest changing your guppies’ water once per week and cleaning their tank whenever necessary to remove waste, dead matter, and food residues. This will minimize your guppies’ stress and help them live longer and happier.
  • Avoid aggressive tank mates – Always pair your guppies with peaceful tank companions. The ideal guppy tank mates should be of similar size, friendly, and share the same water requirements. Furthermore, you should decorate your guppies’ tank with plants and other decorative elements. These elements will break the line of sight between the fish and provide valuable, safe spaces to help them feel comfortable.
  • Early disease diagnosis and treatment – Guppies will always display foretelling signs when experiencing an infection or disease. You should always check your guppies’ behavior and investigate their situation if they display unexplained signs of stress. Quarantining them should be the first step, as this helps with the treatment and contains the disease in case it’s contagious.

Remember, prolonged stress will affect your guppies’ immune system, causing them to experience health problems along the way. Fortunately, you now have several ways to prevent that.

– Heavy Breathing

If your guppy showcases heavy breathing, consider the likelihood of them having been poisoned. You can tell when your guppy has trouble breathing. The fish will open and close the mouth rapidly as if it’s struggling to catch some air. The affected guppy may also swim near the water’s surface since the dissolved oxygen levels in the water are higher in that area.

There are several issues linked to fish poisoning:

  • Ammonia and nitrites – Ammonia and nitrites are the byproducts of organic matter decomposing in the water. This includes fish waste, dead plants and fish, decaying food residues, and so on. Both these chemicals are lethal to your fish, so you should keep their levels at 0 at all times. This is relatively easy in a properly cycled tank and with a steady maintenance routine. You should always monitor your guppies’ water quality to prevent those chemicals in the water in the first place.
  • Chlorine – Chlorine poisoning is actually quite common in the fish industry. Especially among novice aquarists who are oblivious to the dangers. Chlorine doesn’t form naturally in the tank water, like ammonia, but it always comes from outside. You should blame tap water for that, as many inexperienced aquarists use chlorinated water to clean tank equipment and perform water changes. These are deadly mistakes since chlorine is poison to fish.
  • Heavy metals – This one is trickier but equally dangerous. The most common reason for higher heavy metal content in the tank water is contaminated equipment or decorations. Your fish will display visible physical discomfort in the presence of heavy metals and will soon showcase severe health problems as a result. Your intervention is vital in this case.

The solutions:

(Un)fortunately, your guppies will display a variety of other symptoms when facing chemical poisoning. The heavy breathing is but one of them. Other relevant signs include lack of appetite, hiding behavior, difficulties swimming, red or bleeding gills, etc.

The treatment unveils in a similar manner in almost all cases. You simply quarantine the fish in a hospital tank, provide impeccable water conditions, perform daily partial water changes, and treat the fish for any infections.

However, preventing these issues always rank higher in terms of gained value. When it comes to prevention, consider the following:

  • Prevent ammonia and nitrite buildup – The prevention process involves performing regular tank maintenance and weekly water changes. You should also remove excess fish waste and food residues to prevent them from decaying in the water. Maintaining your tank and water in pristine condition is enough to prevent this problem for the most part.
  • Avoid tap water – Tap water contains high levels of chlorine which is toxic to all fish. Chlorine is necessary to sterilize the water and make it safe for humans, but this ranks as a toxin for aquatic life. Avoid tap water altogether if possible, including when cleaning the tank equipment and performing water changes. Use alternative water sources like RO/DI or other chlorine-free water types. You can also use a dechlorinator to remove chlorine from the tap water, making it safe for use.
  • Ensure optimal chemical filtration – A filtration system based on activated carbon is necessary to perform optimal chemical filtration. You can also use water conditioners specifically designed to clean the water of any pathogens and heavy metals and boost your fish’s mucus production. These serve to create a healthier and more stable environment for your guppies, should the situation require them.

– Cotton Mouth Disease

Cotton mouth disease is often mistaken for regular bacterial infections, and this is a costly confusion to make. This condition is mostly deadly and will kill the fish fast when treatment isn’t present. Your guppy will display a variety of symptoms when infected, such as:

  • Keeping its mouth open or moving it as if gasping for air
  • Frayed or ragged fins
  • Changes in body coloring as the fish displays white areas, along with cloudy fungal patches
  • Excess mucus on the gills and head
  • Red or brownish gills, often displaying signs of necrosis in advanced stages
  • Anorexia, lethargy, and hiding behavior

The condition is deadly and can kill the infected fish within 48 to 72 hours. The most common causes of infection include poor water conditions and fish having low immune responses. The latter is common in stressed fish, so re-read the section where we’ve tackled this aspect.

The treatment is pretty straightforward:

  • Quarantine the fish to contain the cotton mouth disease since this condition is highly contagious
  • Lower tank water temperature to 75 °F or slightly lower since the pathogen responsible (Flavobacterium columnare) is most virulent at higher temperatures, usually around 85 to 90 °F
  • Add tank salt to promote healthy mucus production and activate the guppies’ natural healing mechanisms
  • Use antibiotics to counter the bacteria (kanamycin is a good option, and so is nitrofurazone)

Ideally, you want to discuss the problem with an expert fish professional to make sure you’re not screwing things up. Also, monitor your fish’s response to treatment. If the treatment seems ineffective, consider euthanizing the fish.


Guppies will face a variety of health issues along the way. Fortunately, you can minimize their impact or, even better, avoid them altogether by sticking to a robust prevention system. Such a system typically comes down to providing guppies with healthy waters, a stress-free environment, balanced nutrition, and adequate care over the years.

This approach will keep your guppies healthy, boost their coloring, and even prolong their lifespan considerably.

avatar Noah
I’m Noah, chief editor at VIVO Pets and the proud owner of a playful, energetic husky (Max). I’ve been a volunteer at Rex Animal Rescue for over 2 years. I love learning and writing about different animals that can be kept as pets. read more...

Questions and Answers

Please reply soon

😰 my guppy seems to be choking on something or i don’t even know she has been laying at the bottom breathing really fast and opening her mouth fast and then closing it. I see something grey and white inside her mouth but I’m not sure if that’s just part of her mouth so I don’t want to risk trying to take it out. I do have black and grey gravel in my tank or maybe she ate one of her fry. Iv put some melafix in the water but it doesn’t seem to be doing anything. Please help!

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