Why Do Fish Keep Dying: Dissecting 8 Key Reasons – 2020


It’s an advantageous moment when you add your fish to your tank for the first time and watching them adjust to their new home.


Why Do Fish Keep Dying- It’s not so great the next morning when you find that your new little friend lifeless – it’s like getting hit by a ton of bricks. Wondering why fish die unexpectedly and suddenly?


All creatures are genetically predisposed to die (discounting the ‘immortal’ jellyfish, of course) – it’s a natural process that we have very little control over. And fish happens to be a lot more fragile when they’re not in their natural habitat. You’re essentially fighting an uphill battle to keep your fish from dying – and the clock is ticking the moment you introduce them to a home tank.


From the temperature to the complex water chemistry requirements, to the type of tank mates, there are many moving parts in fishkeeping to the food they eat. 


While we can’t stop their biological clock from running its course – we can identify the factors responsible for fish death and do something about it.


In this blog, we’ve narrowed down our pick of the 8 most common reasons why fish keep dying.



aquarium temperatures





Rapidly Fluctuating Aquarium Temperatures 


Your home aquarium should recreate the same water temperature range as the natural habitat of your fish. The good news is that it’s relatively easy to mimic their natural habitat.


This isn’t to say that fish don’t survive rapid fluctuations. After all, the oceans, rivers, and lakes aren’t always at a constant temperature.


But we’ve noticed that the moment you allow the temperature to change beyond a certain range, or if you allow the temperature to change too rapidly, your fish’s health starts deteriorating.


The shock will become a source of stress for your fish, leaving their immune systems vulnerable to attack from parasites and disease.


Outside factors like lighting, direct sunlight, and heat sources can contribute to rapidly changing temperatures. This is why it is important to place your tank in a quiet, dark corner of the room away from direct sunlight. The idea is to stop rapid external weather fluctuations from affecting the tank.


Just take one look at the recent data of fish populations rapidly depleting in the ocean because of global warming – the temperature only changed by a few degrees, and that’s all it took to wipe entire fish colonies.


Solution: Buy an Aquarium Heater to Stabilize the Temps


The most effective way to avoid sudden temperature drops is to invest in a reliable aquarium heater.


The cheaper ones will either malfunction or break, costing you more money in the long run, and are more likely to give you inaccurate readings. This makes maintaining the right temperature in your tank quite difficult.


Experts recommend titanium heaters because they’re highly durable and resistant to both salt and freshwaters. The best place to install the heater is near the filter. This allows the heated water to be distributed evenly throughout the tank and minimize hot and cold spots.


Water Chemistry


Maintaining the ideal water chemistry is the single most important aspect of sustaining a thriving tank. It sounds simple, but most hobbyists will quickly discover that it can be challenging.


When we talk about water chemistry, we refer to the water’s chemical properties in the tank. These properties include carbonate hardness, general hardness, pH value, ammonia, nitrate, and nitrite levels.


Water has an array of chemical and mineral characteristics that you should test regularly.


It is essential to regularly test the water for pH, ammonia, nitrites, nitrates, and water hardness levels. This is done by purchasing aquarium water testing kits. You can easily find these at pet stores near you or purchase them online. There are many types of aquarium testing kits. We recommend purchasing a ‘master’ kit that can test the water for all properties.


Below is a general guideline on what the water levels should be. The actual chemistry depends on the type of fish you’re going for. 




The pH value is a measure of the acidity or basicity of a solution. The pH scale is measured from 0 to 14, with 0 being the most acidic and 14 being the most basic your water can be.


Freshwater fish thrive best at a pH value between 6.7 to 7.7. Most saltwater fish tanks require a pH value of between 8.0 to 8.4 for the best results.


If your water’s pH level is unacceptable, you can alter this value by treating it with chemical products from your local pet stores. It could be argued that maintaining a consistent pH value is more important than maintaining the temperature – and is one of the first steps towards consistently high water quality.


Ammonia Levels


Ammonia is one of the most toxic chemicals to fish and can quickly kill your entire stock. Unfortunately, it is produced naturally in all fish tanks as waste and uneaten fish food break down.


The only acceptable level of ammonia is 0 ppm (parts per million). You can bring the ammonia levels down with the help of many products from your local pet store, but until you’ve completed the tank’s nitrogen cycle, it will always be present in small tanks.


Until you’ve established the Nitrogen Cycle, it is important to add hardy fish such as Mollies.




Nitrite levels rise when bacteria in your tank break down the ammonia. Although nitrates are less harmful than ammonia, they are still very toxic to freshwater fish.


New tanks are especially susceptible to high levels of nitrites once the ammonia is taken care of. The ideal level for nitrites is 0 ppm. To remove nitrites, you’ll have to partially change the water (by about 25% to 30%) weekly.




Nitrates are a secondary product of the Nitrogen Cycle and are generated after the breakdown of nitrites. They are not as harmful to your fish (in lower quantities), and their presence indicates that you’ve completed the cycling of your tank.


If the nitrate levels get too high, your fish will experience stress. The ideal nitrate levels should be between 25 to 40 ppm.


If you want to regulate the nitrate level, you will have to perform regular water changes.  


General Hardness


The total dissolved minerals (magnesium and calcium) in the water are known as the water’s general hardness. Soft water has low amounts of dissolved minerals. Hard water has a higher amount.


The tank should have a water hardness of between 5 to 12 degrees (DH). If the reading is too low, consider changing the water to raise the hardness levels.


Alkalinity Tests (to measure carbonate hardness)


Alkalinity tests determine the tank’s ability to maintain a constant pH value. It is a good indicator of the water’s quality levels. Low alkalinity means that the pH value will fluctuate more easily, and that will almost always manifest as stress on your fish. Low alkalinity levels will stunt the growth of live plants in the tank.


The optimum alkalinity level should read between 6 to 12 degrees (dKH) for both saltwater and freshwater fish.


  • Feeding Frenzy



Most fish cannot tell when they’re satiated and could eat forever – if they could help it. Overfeeding is one of the fastest ways to cause serious health risks to your fish that could lead to their deaths.


It also generates a lot of biological waste that can alter the water’s chemistry. Not to mention the fact that leftover food also disrupts the water quality.


How to tell if you are overfeeding your fish?


  • The filter system is getting clogged by uneaten food.
  • Small white and brown coloured worms are thriving in the water. Make sure to treat your tank right away. Also, worms will eat your fish’s eggs.
  • Milky water as a result of uneaten food and decaying plants. Try not to change water changes too quickly unless it becomes necessary. Stop feeding your fish for a few days, and the water will clear up in 2 to 3 days, resume feeding right away.
  • The fish’s digestive system is not equipped to deal with too much food.

Another major issue for beginners is not feeding their fish a proper diet. An incomplete diet can result in vitamin and nutrient deficiencies and the onset of debilitating diseases such as improper growth, a weakened immune system, and even death.


You will have to research your fish’s specific nutritional requirements to ensure you’re feeding them the right foods that can satisfy their nutritional needs.


Most fish have varied and specific diets that you need to follow. Not all fish will live on blood worms alone. Some will need superworms. Others will like dry fish food flakes. Make sure to check up on their care sheet to know what food regiment to put them on.


  • Lack of Aquarium Husbandry



Happy and healthy fish require clean and safe aquariums. But if you go overboard with the cleaning, you could end up destroying all the beneficial bacterial colonies that play a role in regulating your fish’s bioload.


It is not advisable to start over unless your aquarium is too far gone. There are many ways of cleaning your tank without displacing everything or causing distress to your fish. All you need are a few cleaning supplies, and your aquarium will become clean again.


Here’s a list of things you will need to clean your tank:


  • Water siphon
  • Algae scraper
  • Bleach
  • Filter brush
  • Paper towers
  • Chlorine remover


Make sure to clean your tank in the following order:


  • Start with the inside glass
  • Clean the decorations (such as rockwork and plants)
  • Clean the substrate
  • Now clean the outside of the tank and any other fixtures
  • Finally, clean the filter


Tank Mates Are Incompatible



Another common problem is placing incompatible tank mates in the same tank. If you’ve got too many fish in a tank, it may be highly probable that one or more species will be swimming in water that isn’t at the right temperature for them.


If you decide to adjust the temperature to what they are comfortable with, you risk upsetting the entire aquarium.


A freshwater fish will die in a saltwater aquarium. Similarly, fish species that prefer cold water will likely die in hot water.


Not to mention the fact that some fish don’t get along. Aggressive species like the Cichlids will give docile fish a harder time and may even decide to eat them. So be careful what tank mates you choose. 


Refer to the care sheet of the fish to avoid this issue. Your local pet shop should be able to provide you with the necessary instructions needed to keep your fish in a safe environment.


If not, you can always find care sheets with a quick Google search. At Fish in Aquarium, you can browse from over 50 care guides covering saltwater and freshwater fish species, including mollies, damselfish, and gobies.


  • New Tank Syndrome


Fish do not like crystal clear, pristine waters. It may sound ironic, but it does make sense when you think about it. Is their natural habitat in the wild clean? Not really.


This means that your aquarium shouldn’t be 100% sterile either. It would help if you had beneficial bacteria that are vital to maintaining the right mineral levels in the tank.


The ‘good’ bacterial colonies play an integral role in breaking down the bioload your fish will produce during the course of an average day. If there are no bacteria, or if there are not enough bacteria in the tank, your water’s ammonia levels will begin to spike. And that will prove to be too toxic and could result in fish death.


The Aquarium is Too Small


Do your fish keep dying, but water is fine? This is because most beginner hobbyists are known for overcrowding their aquarium with too many fish. Your fish need their space.


Imagine the stress of being moved from the vastness of the deep blue sea into a home tank that is not only overly restricted but also has unusual species they’ve never encountered – it’s like a death sentence to fish.


It is also worth noting that the more fish you add to your tank, the more bioload they produce. If the rate of waste accumulation exceeds the breakdown rate from bacterial colonies – the water will become too toxic and kill your entire stock in the process.


A small tank may not have enough dissolved oxygen for all fish and plants, causing some of them to suffocate and die a slow, agonizing death. If you’ve purchased juvenile fish, they’ll experience stunted growth because of the tank. 


For this reason, beginners should adopt bettas and goldfish before moving on to tougher species like Damselfish and Clownfish. A single betta can live in a 3-gallon tank. But a clownfish will need at least 20 gallons to live.


Now is also a good time to tackle the fishbowl myth. Simply put, a bowl cannot provide adequate living conditions for any fish, big or small. Are your betta fish dying for unexplainable reasons? It’s probably because you’ve kept them in a tank that isn’t large enough to provide them with a healthy home. 


The Fish Were Unhealthy, to Begin With


It is entirely possible that your fish was sick, stressed out, or in poor condition before they were introduced to your aquarium. And even the best tank conditions may not be enough to keep them alive. This is why it is vital to understand the symptoms of common diseases for the fish that you want to keep as a pet.


In most cases, the logistics of fish transportation are enough to take a toll on your little pets – and may never subside.


To prevent this from happening, try to only buy your fish from experienced and reputable dealers or breeders. Select only the healthiest fish in the best condition so they can withstand the stresses of transportation and be introduced into a new tank. 


You Haven’t Established the Tank’s Nitrogen Cycle Yet.


A big reason for fish dying in a new tank is the absence of a biological filtration system.


This is commonly done through a biological process known as the Nitrogen Cycle. The Nitrogen Cycle creates a natural filtration system and bacterial environment that is essential to your fish’s health.


It can take two weeks or even two months for this cycle to complete. At the end of the Nitrogen Cycle, the bacteria will have removed your tap water’s harmful elements, thus giving you ideal water quality for fish.


How Do Fish Die?


Most fish dying instances can be put down to the fishkeeper making a mistake or not knowing any better. Perhaps one of your fish got affected by a common fish disease, and it spread around, thereby killing all your fish in one fell swoop.


If you did everything right and your fish still died, it could be due to natural causes. Fish can die of old age, parasites, severe weather, and even injury. 


To ensure your fish’s survivability, you have to learn their specific care requirements and recreate their natural habitat in your home tank.

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