Why Are My Guppies Lethargic?

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Most novice guppy keepers are oblivious to the various factors that will influence a guppy’s wellbeing. Your guppy may experience stress, get more aggressive occasionally, look for hiding, and display low energy levels for no apparent reason.

But that’s only at first glance. Going in-depth, we learn that every behavior has some triggering factor behind it.

Today, we will discuss lethargy in guppies, highlighting the causes and pinpointing the potential fixes.

So, what are the potential causes for your guppies’ visible lethargy? Here are the most common ones:

Temperature Drop

As tropical fish, guppies will tolerate temperatures ranging between 72 to 82 °F. They do allow for some variations outside these thresholds, but not by much. A guppy may withstand 60 °F, but only for a short while.

Prolonged exposure to temperatures below 65 or, even worse, will affect your guppy and even kill it as a result. Cold waters will affect the guppy’s immune system and heavily influence its behavior. The guppy will appear more lethargic as it will try to conserve energy in an adverse environment.

You may also see the guppy’s appetite go down while showing an increased predisposition towards infections and disease. The latter is the result of the fish’s immune system being sabotaged by the dangerously low temperatures.

To prevent that, monitor the tank’s temperature constantly, especially if it’s placed in an area with fluctuating temperatures. You may also need to get a water heater if the environmental temperature tends to fluctuate too aggressively.

Lack of Oxygen

The water’s oxygen levels may drop for a variety of reasons. These include overcrowding, poor tank maintenance, algae, and plant overgrowth, lack of periodical water changes, etc. Severe drops in oxygen levels will affect your guppies and even lead to asphyxiation and death.

Fortunately, you will notice tale-telling signs warning of the problem. Some of them include your guppy gasping for air at the water’s surface, erratic swimming, and low levels of activity and lethargy.

Guppies will become lethargic in an effort to preserve energy since movement forces their muscles to demand more oxygen. If you notice your guppy gasping for air and appearing lethargic, verify the oxygen levels and perform an immediate water change.

You should also trim the plants, clean the algae, and remove any fish and food waste that may pollute the water. Avoiding overcrowding is also key in keeping your fish population healthy, active, and overall peaceful.

As a bit of general advice, every guppy should have a minimum of 2 gallons of water available. I would recommend slightly more to remain on the safe side.

pH Fluctuations

The ideal pH range for guppies stands between 7 and 8. Go lower than 5, and your fish will begin to feel the change immediately. The lower the pH drops, the more acidic the environment will become, which can prove deadly to your guppies. A low-enough pH can burn your guppy’s gills and skin, leading to a swift and painful death.

You should also remember that many things will trigger pH fluctuations in the tank, including water changes, various tank wood, and plants, adding new fish to the environment, etc. The goal is to keep the pH levels in the 7-8 range, which you can achieve via specific methods.

You first need to use a water test kit designed to assess the water’s pH level. Then you can play with its values like so:

To increase pH levels – Use an air pump to eliminate excess CO2, add limestone to the water, aerate the tank, or use an alkaline buffer.

To decrease pH levels – Add peat moss in the water filter, use a water softener, or mix an acid buffer in the water.

Ammonia or Nitrite Poisoning

Ammonia and nitrite poisoning are common in poorly maintained tanks. The first symptoms will generally appear immediately, with the fish gasping for air, displaying blood-red gills, and exhibiting lower appetite.

The fish will soon stop eating, swim at the bottom of the tank, and even display bloody patches on its body and general lethargy. Ammonia poisoning can result from various sources, including dead animal and plant matter, festering in the tank, overfeeding, adding tap water with high chloramine levels, etc.

It’s necessary to monitor ammonia levels constantly in the tank to prevent this condition. Remember, ammonia levels can spike fast and become deadly within several hours.

You can decrease the ammonia levels in a tank by:

  • Performing partial water changes
  • Cycling the tank
  • Rely on a potent filtration system
  • Boost the amount of beneficial bacteria in the tank
  • Increase the tank’s size, etc.

High Nitrate Level

Whatever increases the ammonia levels will also increase that of nitrates. The ideal nitrate level should remain below 25 ppm, but it generally depends on the tank’s size, the fish breed, and several other factors.

Generally, a level of nitrates above 10 ppm will promote algae growth, lowering the water’s oxygen. That being said, your guppies will begin to show significant discomfort at 100 ppm and higher. Prolonged exposure to more than 50 ppm will also cause visible symptoms, including erratic swimming, lack of appetite, hiding behavior, and lethargy.

The high nitrate levels will eventually render the fish vulnerable to disease and parasites and inhibit its reproductive abilities.

You can counter this problem by performing regular water changes, keeping live plants in the tank, avoiding overfeeding, and cleaning the tank regularly.

Diseases or Parasites

Guppies may struggle with a variety of parasites and diseases, each with their specific symptoms and effects. All disorders, however, will trigger similar behavioral responses. The fish may become more secluded, seek hiding, show lethargy and lack of energy, and even stop eating.

You will also notice disorder-specific symptoms, varying depending on the nature of the disease. These could include white spots on the body, red gills, damaged fins and tail, body sores, etc. At this point, you need to diagnose the fish’s condition to address it properly.

The first step would be to quarantine the fish, then diagnose the condition, and treat it via specific procedures. These may include regular water changes, using antibiotics, cleaning the tank more often, increasing oxygenation, etc.


Overfeeding is a classic problem for most novice guppy keepers. Guppies tend to eat more than they should, simply because their brains are hardwired to find food constantly. This isn’t a problem in their wild environment, where live food is rather scarce, and guppies need to struggle to find it.

The same problem no longer exists for aquarium guppies, who will receive as much food as their keeper provides. The problem is that most novice guppy keepers believe that feeding their guppies as much as they (guppies) want is a good thing. It’s not.

Guppies should eat, at most, once or twice per day. Anything above that risks of becoming overfeeding, which will affect guppies in the long run. They will experience lethargy, bloated abdomen, constipation, etc. Continuous overfeeding will also cause the ammonia levels to spike, as the unconsumed food will decay in the water.

To prevent that, I advise only feeding your guppies once per day. This is enough to keep them healthy, active, and happy in the long run. You may even feed them twice, once in the morning and once in the afternoon, so long as you control the amount of food they’re getting.

Only offer guppies as much food as they consume in 1 minute or less.


Bullying is common in a tank with different fish breeds of various sizes, multiple males, or with overcrowding issues. Bullying isn’t necessarily a major problem unless it becomes repeating behavior.

Male-only aquariums will experience bullying constantly, especially if the number of females is lower than it should be. Guppy males can be quite territorial at times and will bully other males for the right to more food, mating, and even only for being males. That’s because males are predisposed to higher aggression thanks to their elevated testosterone levels.

  • To reduce the bullying:
  • Increase the tank’s size
  • Increase the number of females (you should have 2-3 females for every guppy male)
  • Make sure all fish are of similar sizes (the larger fish tend to bully the smaller ones)
  • Provide enough food for all fish
  • Make sure that the different fish are compatible as tank mates

Use of Liquid Fertilizer

Liquid fertilizers are useful for providing nutrients to aquarium plants. They are generally safe for fish, shrimp, snails, and other tank inhabitants, but not always. Use them excessively, and your fish may experience health issues as a result. You should only use liquid fertilizers once per week for a 55-gallon tank, for instance.

I should also mention that some fertilizers are only meant for plant tanks-only. You shouldn’t use them in fish tanks since they can cause severe side-effects and even death.

To prevent problems like these, always read the product’s label to understand where and how to use it.

Use of Medicine

Certain medications can harm your fish, including fish-specific medication. This problem often arises due to people treating sick fish in the same tank with the healthy ones. This can cause healthy fish to develop kidney problems, leading to various health issues along the way.

This is why you should always quarantine sick fish before treating them. You do so both to contain the disease and prevent the medication from hurting the healthy specimens.


Guppies can appear lethargic for a variety of reasons. Some are mild, while others can cause life-threatening conditions. Knowing this behavior’s underlying causes can literally make the difference between life and death.

I suggest you quarantine the lethargic guppy as soon as you notice it. This way, you will protect the rest of the guppy population and address the guppy’s problems more effectively. Always consult with the veterinarian to make sure you provide your guppy with the ideal care and treatment.

Updated: December 10, 2021
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