What kills bacteria in a fish tank

What kills bacteria in a fish tank? 8 Main Reasons

A balanced and healthy aquarium relies on bacteria to decompose fish waste, plant matter, and other organic trash that accumulates in the tank. They maintain the water crystal clean and prevent the accumulation of harmful ammonia and nitrite. It takes time for these bacteria to become established in the filter medium and on solid aquarium surfaces.

What kills bacteria in a fish tank? The majority of aquarium microorganisms die from poor water quality. The aquarium occupants are affected by different factors, including pH, water temperature, and water quality. The ecosystem of a fish tank can be badly harmed by exposure to chlorine or chloramine in the water. As a result of their disinfecting properties, these compounds are frequently added to municipal tap water. Harmful microorganisms are eliminated, and the use of these compounds hampers biological filtering.

Eight main culprits of bacteria death in aquariums are:

  • High Water pH  
  • Common Causes of High pH
  • Temperature Changes
  • Tap Water Pollutants
  • Excess ammonia
  • Low oxygen levels
  • Overstocking
  • Antibiotics

Consequently, many water quality issues may arise. Bacteria, bacterial infections, are the second most frequent ailment in aquarium fish after parasites. Bacteria live in different places in a mature fish tank. You can find them in the filtration system, substrate, glass walls, and decorations. They are crucial in balancing the delicate nitrogen cycle and maintaining water quality. Unfortunately, these organisms are vulnerable to certain chemicals and medications

What kills bacteria in a fish tank

Bacteria can withstand moderate levels of salinity. Hence, you will not harm them by dosing the tank water with low concentrations of aquarium salt. However, higher concentrations of aquarium salt can affect the bacteria and disrupt the natural filtering mechanism of the tank.

Unfortunately, chlorine and chloramine are not only harmful to aquarium fish, but they can also have negative effects on the overall aquarium system. These chemicals also inhibit biological filtration and destroy helpful microbes. As a result, many water quality issues, such as dangerous ammonia spikes, may arise.

High Water pH  

Bacteria are extremely sensitive to pH fluctuations, and fast pH transitions can severely damage aquarium occupants. Improper or excessive pH fluctuations can even harm the overall health of fish tank creatures. Any change in the pH of water than normal may affect the growth of the bacteria.

Common Causes of High pH

Numerous biological processes can alter the pH of your aquarium. Since additional activity, including algae, fish, and invertebrates, delivers more hydronium ions into a solution, pH decreases are more frequent. Typically, high pH derives from the source water. The primary water pH will be based on your area and the presence or absence of home filtration.

Unless you are beginning with RO water, you must do your experiments to determine the dosage of pH-adjusting additives if you intend to use them to change the pH of your water to accept specific species.

Temperature Changes

The fish tank’s inhabitants are also particularly vulnerable to temperature changes, as fluctuations can weaken their defenses and ultimately kill them. When stressed, bacteria become more vulnerable to infections like ich and others. It is important to avoid temperature fluctuations and keep the water at a consistent temperature.

A hazardous lack of oxygen is caused by the inability of warmer water to “hang on to” it. Hypoxia, often known as oxygen starvation, is one of the initial effects you’ll notice. Fish and bacteria in warm water may appear to be breathing heavily and thrashing about at the surface as if attempting to get fresh air.

Tap Water Pollutants

Water from the tap may contain toxic compounds to microorganisms and fish. Chloride, chlorination, and toxic substances are typical tap water contaminants. Without suitable water purifiers, chlorine and chloramine can damage fish gills and destroy beneficial bacteria that offer biological filtration.

Excess ammonia

Ammonia can be a big problem for aquarium species. It can quickly kill delicate fish species or make for a slow, painful death for more hardy fish species and bacteria. Ammonia and nitrites can be managed properly with the proper equipment and knowledge.

Low oxygen levels

Oxygen is essential to the survival of aquatic organisms. Fish, bacteria, shrimp, freshwater snails, crabs, etc., require oxygen for metabolic activities. It also maintains their health; if they are deprived of oxygen for an extended period, they will expire. Similar to other living organisms, they take in oxygen and exhale carbon dioxide.

The difference is that oxygen is less accessible to aquatic species than terrestrial ones. Unfortunately, the low oxygen level is hazardous to the health of all aquatic animals. It will restrict their movements and force them to swim or crawl to the water’s surface to breathe to survive.


When a tank is brand new, the bacteria have not been colonized. Adding as many fish as possible when stocking your aquarium is very tempting. However, doing so will lead to water quality issues that will negatively affect your aquatic pets’ health and kill bacteria and other water species in a fish tank. Suppose you believe your aquarium is overstocked with fish, resulting in spikes of ammonia, nitrites or nitrate.

In that case, you should immediately do a water change to bring those two compounds down to safer levels. Make sure you also use a water conditioner that detoxifies ammonia and nitrite for a limited amount of time for the entire volume of your tank.


Some antibiotics will kill most bacteria in your tank without affecting algae or your fish (well, maybe it will kill their gut bacteria too, which is not good). And then, the fish will die of ammonia poisoning since there will be no more bacteria consuming ammonia and a lot of dead bacteria becoming ammonia.

Why Are Beneficial Bacteria Important?

Due to a shortage of these bacteria, newly installed aquariums may experience severe increases in ammonia and nitrite from fish waste. This occurs when more fish or food are supplied than can be processed by the present bacteria. In what is known as the Nitrogen Cycle, fish excrete ammonia, which is then broken down by beneficial “nitrifying” bacteria into nitrite and, eventually, nitrate.

Ammonia and nitrite are both extremely hazardous to fish. When setting up a new aquarium, it is important to add fish gradually and feed them sparingly. This gives good bacteria time to settle in the tank and filter. In aquariums that are already set up, solid fish waste, uneaten food, dead plant material, and other organic waste can cause the water quality to get worse and the pH to drop.

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