Are High Nitrites Dangerous To Fish?

When you first start setting up an aquarium you will probably hear the words, ammonia, nitrites, and nitrates.

When cycling a tank (basically getting your water levels safe) you will need to control all of these for a healthy tank. So, are high nitrites dangerous to fish? Yes, they are. High levels of nitrates and ammonia can be just as lethal as well. This is why you have to control them and keep a close eye on their levels to maintain a safe environment in your tank.

What Are Nitrites?

Nitrites are a toxic type of waste that forms when ammonia is present in your water and not to be confused with nitrates. Nitrifying bacteria start breaking down all the ammonia thus causing nitrites. Although nitrites are harmful to your fish, you do actually need them as part of the nitrogen cycle. When setting up a tank for the first time you need to get everything established. This is called cycling your tank.

So basically, fish tank waste such as decaying plants and fish poo breaks down and lets off ammonia. Nitrites then form and eat away at the ammonia turning it into nitrates. Once the nitrates are active your tank is fully cycled. This can take up to 8 weeks. Although there are treatments available to speed up the process if needed. I, like many other aquarists, prefer to do it the organic way though even if it takes a bit longer. I don’t like putting any type of chemicals in my setups if I can help it. The video below explains it in better detail.

What Causes High Nitrites In A Fish Tank?

There are 5 main reasons why you may have high nitrite levels in your tank.

Leftover Fish Food

Leftover fish food is a very common problem in beginners aquariums. It’s quite surprising how little amounts of food the average fish actually consumes. When a fish is feeding it will usually eat all it needs to in a couple of minutes. Keep a close eye on your fish during feeding times, if there is food leftover after this time it will be worth feeding less the next time. By doing this it should dramatically reduce the amount of decaying food in your tank.

Decaying Plants

If you have live plants in your tank you will know that as they grow leaves and stems will fall off at some time and start decaying away. Every time you notice any loose bits of plant be sure to get them out of the tank. One of the easiest ways of doing this is with a gravel vac. They are cheap as chips and can also come in handy when doing water changes.

Fish Waste

Fish waste is certainly one of the biggest causes of nitrites in your tank. What goes in obviously has to come out somewhere. If you have a good size community tank with a fair few fish this can soon become a problem. As with the decaying plants, a gravel vac will make short work of this and have your substrates clean and poop-free in no time.

Dirty Filters

Dirty filters are not good in any aquariums. That being said you need to be careful when cleaning them. Filters tend to house good bacteria that kill off things such as ammonia and nitrites. For this reason, you want to be keeping this bacteria and not killing it off. Be sure not to clean your filters in tap water, that is the best way of killing your good bacteria. Instead, take some of your aquarium water and clean your filters in that. This will keep the bacteria that you need alive and well.

Dead Fish

Leaving Any dead fish in your tank is a big no-no. A dead fish will give off all sorts of nasties that will affect your remaining fish. Be sure to get rid of them as soon as possible. Closely inspect the dead fish to see if you can figure out why it died, if it is from disease then treat accordingly. I would advise doing a 50% water change as well.

Can High Nitrites Cause Death In Fish.

Yes, 100% high nitrates can and will cause major stress to your fish, almost certainly resulting in death if left untreated. All tanks will end up with nitrites present, they are needed to complete the nitrogen cycle. The trick is to keep the levels down to a manageable amount. A test kit for your tank is a must. Personally, I would steer clear of the strip tests and go for the water sample test kits instead. They may be a little more expensive, but in general, they will give better results. This is the best way of monitoring your parameters. Alternatively, most aquatic stores will do your testing for you. Just take them a sample of your tank water and they will take care of the necessary tests that are needed.

How To Lower Nitrite Levels

If you are unfortunate enough to suffer a nitrite spike, or just suffer with high levels of nitrites here are a couple of things you can do.

A Water Change

If you have high nitrite levels immediate action will be required. A water change should be your first port of call. Personally, I would do a 50% change. Be sure to treat your tap water before adding it back into the tank. Natural spring water is an awesome alternative to tap water. It is a lot purer than tap water and will be chlorine-free. Therefore you won’t be needing to add any chemicals to it to make it safe for your tank.

Stop Feeding The Fish

You don’t have to totally starve your fish, but it is worth halting your feeding for a day or two. This cuts off a food source for the nasty bacteria that turn ammonia into nitrites.

Water Conditioner

Once you have done your water change the nitrite levels should drop a bit. The next step is to add a water conditioner to your tank. Don’t worry, as long as you follow the instructions it will not cause any harm to your livestock. Certain water conditioners are a god send for sorting out conditions such as nitrite spikes. The one I use is Seachem Prime.

Click on the image for more details


Basically, high nitrite levels in your tank are lethal to anything that lives in it.

You really don’t want any nitrites in your aquarium once it is established. There should be enough bacteria in the water to eradicate the nitrites as they form. Regular water changes and clearing up any waste should keep your nitrite levels to 0 if done properly. If you do have a nitrite spike (sudden high levels of nitrites) it needs to be dealt with immediately. It is worth getting yourself a nitrite test kit if you don’t already have one. Happy fish keeping and enjoy your fish.

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