Is 0.25 ppm Ammonia Bad? Will it Kill Fish?

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As an aquarist, I tend to measure the toxins in my tank. This way, I can prevent ammonia spikes, which can be extremely dangerous for fish. However, quite a few times, I measured 0.25 ppm of ammonia, and I wasn’t sure whether it is safe or bad for fish. The more I investigated the situation, the better I understood what levels of ammonia could kill fish.

0.25 ppm of ammonia is typically bad for fish since it induces stress, compromising the fish’s well-being in the long haul. However, in some cases, 0.25 ppm can be tolerated by fish. That is especially true if the pH and the temperature are adequate and if the toxin’s main component is ammonium.

As we move forward, I will teach you how to lower and possibly eliminate unnecessary ammonia in your tank. That includes using the right aquarium filter and commercial products, such as the Seachem Stability Fish Tank Stabilizer (link to Amazon).

Is 0.25 ppm Ammonia Bad For Fish?

Ammonia occurs naturally in environments that have fish. It isn’t a problem in the wild because the water bodies that fish inhabit are large.[1] They dilute the ammonia, making it less toxic. The same cannot be said for aquariums.

Because they are so small in size, at least compared to the average lake or river, it doesn’t take long for ammonia to grow to dangerous levels. But what counts as a dangerous level of ammonia?

Common sense will tell you that anything above 1.0 ppm is bad, which is true. Once you exceed 1.0 ppm, you are just as likely to kill your fish. Those that survive will fall sick. But what about 0.25 ppm? That sounds like a meager figure.

Indeed, fish can survive 0.25 ppm. Well, for the most part, Yes. 0.25 ppm is low enough that you don’t have to worry about ammonia at that level killing your fish. But that doesn’t make 0.25 ppm good. You are better off having 0.0 ppm where ammonia is concerned.

Concentrations as low as 0.25 ppm won’t kill your fish, but they can still induce stress, especially if they are forced to live in such conditions for long periods. Over time, their immune system will lose its potency, exposing them to all sorts of infections and diseases.[2]

This is why many aquarists have argued that there is no such thing as a safe ammonia concentration. Even small amounts can reduce a fish’s lifespan.[3] A professional aquarist will discourage a beginner from panicking if they have 0.25 ppm of ammonia in their tank. 

For someone that is only starting to understand aquariums, 0.25 Pppm is a good figure. But as their experience grows, people will encourage them to bring that figure down to 0.0 ppm to be on the safe side.

How Many ppm of Ammonia is Dangerous For Fish?

Any ammonia level higher than 0.25 ppm is dangerous for fish. Ammonia becomes exceptionally dangerous once it exceeds 1.0 ppm. At that level, changes occur in the gills’ structure, reducing the oxygen diffusion capacity. In the long term, it will either kill your fish or make them very sick.

What is a Safe Level of Ammonia in an Aquarium?

Now that you know that 1.0 ppm and above is dangerous and that 0.25 ppm is relatively acceptable, you might assume that it is quite easy to identify a safe level of ammonia in the aquarium. But that couldn’t be further from the truth. 

You have to take several factors into account, including:

  • Toxicity – Whenever you test for ammonia in an aquarium, the average test kit is designed to reveal the Total Ammonia Nitrogen, which combines the readings for ammonia (NH3) and ammonium (NH4).

Ammonium is ammonia in an ionized state. Ammonia (NH3), on the other hand, is not ionized. Of the two, ammonium is the least toxic. In fact, ammonium cannot kill fish, not even in high concentration. Ammonia (NH3) is the dangerous one.

Testing kits are a problem because they measure the total concentration of ammonia and ammonium.[4] This is an issue because water with a lot more ammonium and a lot less ammonia is less toxic than water with a lot more ammonia and a lot less ammonium.

In other words, a reading of 1.0 ppm might be more dangerous in one situation because the ammonia concentration is so much higher and less dangerous in another situation because the Ammonium concentration is higher. The average testing kit won’t differentiate between the two.

  • Water pH – The pH influences the toxicity of ammonia in an aquarium. Acidic water, with a low pH, has a lower concentration of unionized Ammonia (NH3) and a higher concentration of ionized ammonia (Ammonium). As the pH rises, the concentration of NH3 grows while the concentration of NH4 drops, making ammonia all the more toxic. 

In other words, a fish might tolerate ammonia at a pH of 7.8 but then die at a pH of 8.4 even though the ammonia levels are the same. That is because the higher pH has allowed the concentration of toxic ammonia (NH3) to rise.[5]

  • Temperature – The temperature doesn’t have as drastic an impact on the toxicity as the pH. But it still matters. Higher temperatures tend to produce NH3 in more significant quantities, which, in turn, makes the ammonia in a tank all the more toxic.

For water between 60 and 80 degrees F, the impact of the temperature on the toxicity is small enough that it might not matter in the long run.[6] The same cannot be said for aquariums whose temperature has exceeded that threshold.

  • The Type of Fish – As you can see, you cannot identify the safest concentration of ammonia by merely looking at the ppm. A lethal ppm for fish at a higher pH and temperature might be safe for the same fish at a lower pH and temperature.

This is why the type of fish is so important. Some people think that the solution to this problem is to lower the pH and temperature. That way, you don’t have to worry about keeping the ammonia levels at 0.0 ppm because concentrations like 1.0 ppm can be safe if the pH and temperature are appropriate.

However, as an aquarist, you don’t get to determine the pH and temperature. Every species has a particular range of pH and temperature in which it thrives. And if your fish requires high pH and temperatures, the kind that enhances the toxicity of ammonia, you have to oblige them.

Suppose you lower the temperature and pH to lower the toxicity of ammonia. In that case, you will kill your fish because they cannot survive for long in water whose parameters differ dramatically from what they like. 

This is why you are better off keeping ammonia levels at 0.0ppm. At this level, you don’t have to worry about the temperature and pH. It is the safest ppm for your aquarium, regardless of the type of fish it contains.

How to Lower High Ammonia Levels in Aquariums?

Follow these steps to lower high ammonia levels in your aquarium:

  1. Cycle your tank to turn ammonia into nitrates.
  2. Replace 20 to 30 percent of the water weekly.
  3. Remove pollutants from the water, such as fish waste and leftovers.
  4. Install a filter of at least 5 GPH for every gallon of water in the aquarium.
  5. Avoid overfeeding your fish.
  6. Take gravel and filter media from an adequately cycled tank.
  7. Use aquarium conditioners to facilitate the breakdown of waste organics.
  8. Stick to large tanks to efficiently dilute the ammonia.
  9. Avoid an overcrowded tank by removing a few fish.
  • Cycling – First of all, if your ammonia levels are always high, and it is always a struggle to lower them, you have to consider the possibility that your tank wasn’t properly cycled. Cycling introduces bacteria (Nitrosomonas) that turn Ammonia into nitrites. It also adds bacteria (Nitrobacter) that turn nitrites into less toxic nitrates.[7]

Without this bacteria, the ammonia in your aquarium will remain consistently high. The only solution is to take your fish out so that you can complete the cycling process. It takes several weeks.[8] You can expedite the process by adding filter media from a cycled tank to your aquarium.

  • Water Change – A water change will do wonders for your aquarium. You should change the water regularly. If your test kit has revealed ammonia levels that exceed 0.0 ppm, change the water immediately. You don’t have to wait for the following scheduled water change. 

20 to 30 percent should be enough. Though, you can perform more considerable water changes if the ammonia levels are dangerously high. The aquarium’s size also matters. Smaller tanks require more frequent water changes since the ammonia will accumulate relatively fast.

  • Hygiene – Ammonia typically forms due to the rotting of organic components. That includes fish waste, leftovers, and dead plants. A water change will clean your water. But you are still expected to scoop all the contaminants out. 

Don’t forget to vacuum the gravel. If you fail to remove debris from the tank, any steps you take to lower the ammonia levels will only produce temporary results. Rotten leftovers will gradually release ammonia, lowering the pH as a consequence.

  • Filter – One way of keeping your tank clean is to install a strong filter. The strength of the filter should match the size of the tank. The bigger the tank, the stronger the filter required. As a rule of thumb, the filter should feature 5 GPH for every gallon of water in the aquarium.[9]

The filter will keep some of the debris out of the tank. However, some elements have to be removed by hand. You are also expected to maintain the water changes. You cannot expect the filter to keep the tank clean on its own.

  • Food – Overfeeding will encourage the ammonia levels in the tank to grow. This is because overfeeding compels fish to produce a lot more waste. It also increases the volume of leftovers in the tank. 

If your ammonia levels are too high, give your fish less food, at least for a few days. This will give you the time to lower the ammonia without the fish immediately undoing your hard work by producing more waste.

  • Bacteria – While good bacteria that remove ammonia are introduced by the cycling process, you can add even more Nitrosomonas and Nitrobacter. Do that by taking gravel and filter media from an older, sufficiently cycled tank and adding them to your current tank. The more good bacteria you have, the more ammonia they will convert.
  • Conditioners – The market is filled with chemical products that can remove ammonia in just a few minutes. Chemical products are perfect for emergencies. You can also use them to treat new water before you add it to the tank. 

I personally use the Seachem Stability Fish Tank Stabilizer (link to Amazon). That product contains a blend of facultative bacteria that efficiently facilitate ammonia spikes. All you have to do is pour one capful, which is about five m/L, for every ten gallons of water in your tank.

But it would help if you didn’t rely on them to keep ammonia out of the tank. If your ammonia levels keep spiking, you are encouraged to find a permanent solution. Do not rely on conditioners. It is better to prevent the situation from happening in the first place.

  • Tank Size – Get the most extensive possible tank for your fish. It is challenging to keep ammonia out of a small tank because the toxin accumulates so quickly. On the other hand, a large tank allows the water to dilute the ammonia.

Ammonia will eventually accumulate to dangerous levels if you fail to maintain the aquarium. But you have a lot of room to breathe because it takes so much longer for toxins in large tanks to rise.

  • Overcrowding – It is crucial to avoid overcrowding when it comes to ammonia spikes. If your tank contains too many fish, waste and leftovers will accumulate so rapidly that you will find it hard to deal with the situation. Try to remove excessive fish. If possible, split your community into several tanks.


If you measured 0.25 ppm of ammonia in your tank, your fish would probably do just fine in the short term. However, in the future, problems will start to occur. That is because 0.25 ppm is still too high for fish to handle.

To ensure that your fish remain healthy, I highly suggest lowering the ammonia to 0 ppm. You can do that by removing the water more frequently, installing a proper filter, or using water conditioners to stabilize your water.