The Nitrogen Cycle

Have you ever wondered what the deal is with water changes on your aquarium? You have a filter and it was probably the best one on the market when it was installed, so dumping out water and adding more seems redundant, right?

Or, you have a brand new aquarium and you’ve added your favorite kind of fish right after you set up the tank. But when you check on them the next day they’re all dead. What gives?

Both of these questions have the same answer: The Nitrogen Cycle of the Aquarium.

The Nitrogen Cycle

Let’s break this down by first explaining what the Nitrogen Cycle is. Take a look at the picture below.

When fish eat and breathe any waste that comes from the fish is converted to Ammonia. And just like the concentrated cleaning chemical, high amounts of Ammonia in your aquarium is bad news. Your fish can experience Ammonia burn on their fins and gills and it will quickly decrease their health and longevity.

In an established tank, good bacteria called Nitrosomonas will break down the Ammonia in the water and convert it to Nitrite. While it’s no longer chemically Ammonia, Nitrite is still harmful in any quantity.

Nitrite is then broken down by more good bacteria called Nitrobacter, and converts Nitrite to Nitrate. Nitrate is much less harmful (in low amounts) in your aquarium. When you test your aquarium water a healthy reading for your tank should be Ammonia: 0ppm, Nitrite: 0ppm, Nitrates: <50ppm*.

*ppm stands for Parts Per Million.

The Nitrogen Cycle

Water Changes

While your filter does work to break down the Ammonia and Nitrite in your aquarium by housing the good bacteria in the filter and in the substrate of the tank, it can’t get everything. Eventually those levels of Nitrate will build up to harmful levels.

Which brings us back to Question 1: Why do I need to do a water change if I already have a filter on my tank?

Water changes are necessary for removing water with built-up levels of Nitrate. In nature, this water would be naturally filtered out by aquatic plants and small bodies of water like a river or a stream leading away from the lake or pond. However, in an aquarium the natural filtration processes have to be done by hand.

A good rule of thumb is to change out 30% (roughly one third) of the aquarium water and replace it with clean dechlorinated water once a month. If you don’t have access to Reverse Osmosis water to refill your aquarium, then tap water treated with tap water conditioner/dechlorinator that is the same temperature as your aquarium water will do just fine.

Tip: If you’re in the warmer months of the year and your aquarium seems to be evaporating faster than usual, you can top it off with clean dechlorinated water. If you have a saltwater aquarium, be sure to top off with fresh water and not saltwater. The salt gets left behind during evaporation, so it won’t change the salinity of your tank if you top off with fresh water, but it will spike the salinity if you top off with saltwater.

New Tank Syndrome

Now on to Question 2: Why are all my new fish dead? It’s a brand-new tank! Well, there’s a reason it’s called “New Tank Syndrome.”

New Tank Syndrome is one of the most common issues that new (or impatient) fish hobbyists run into. An aquarium that has just been set up needs time to build up the beneficial bacteria in the filter and the substrate. Otherwise what happens is a big spike in Ammonia from the fish waste, and there’s no beneficial bacteria to break it down and convert it through Nitrite to Nitrate.

Experienced hobbyists know how to seed their filters by literally taking the biological part of the filter media from an established tank and transferring it to a new one. This piggy-backs the beneficial bacteria from the established tank to the new one.

But if you can’t grow your own beneficial bacterial, store-bought is fine. There are bacterial and chemical additives that are made specifically to jump-start the Nitrogen Cycle in a new aquarium and avoid New Tank Syndrome.

Your new aquarium should cycle for a week or two with the beneficial bacteria building up without fish or other livestock present. You should test your water parameters about every 2-3 days to make sure it’s cycling correctly. If you can, record your testing data. You’ll be able to see when your tank goes through a big Ammonia spike, then it will drop as the bulk of it is converted to Nitrite and then on to Nitrate. When you test your aquarium water it should be Ammonia: 0ppm, Nitrite: 0ppm, Nitrates: <50ppm.

When your water parameters are within acceptable ranges, then you can slowly start to add fish. Another big factor in new tanks “crashing” is because there are too many fish, inverts or corals added at once. The Ammonia spikes and the bacteria colonies that are present in the aquarium can’t handle the bio-load and the new additions die.

It’s best to wait a week or two in between adding new livestock to the tank. The best way to know for sure is to test your water parameters to make sure they’re within acceptable ranges before you head off to the local fish store.

But water changes aren’t the only things you have to stay on top of. Experienced aquarists know that there’s a whole host of maintenance that comes with an aquarium. Click here to see what other water parameters need to be monitored in your reef aquarium.

Making sure that you can handle the work required of an aquarium is an important part of being a hobbyist. Click here to find out what aquarium type is right for you.

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