How Long for a New Filter to Cycle in an Established Fish Tank?

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There comes a time when any fish enthusiast is looking to expand his or her hobby. However, setting up a new aquarium from scratch is a royal pain in the butt. You can’t dump everything inside the tank and just go.

You have to complete the nitrogen cycle first. It’s a long, slow, and frankly boring process. Luckily, you can speed up the cycle in a lot of simple ways.

In this article, we’ll look at one of the most common, quick, and effective methods— seasoning your new filter. This method requires cycling a new filter in an already cycled tank, usually alongside an established filter.

I’m going to cover everything you need to know about this method in detail. Keep reading to learn more about how it works, how to do it, and how to troubleshoot any potential issues. Let’s get started!

Can You Have Multiple Filters in Your Tank?

Having more than one filter in your tank is called “redundant filtration”. Typically, one filter is powerful enough to provide all the filtration power for one aquarium.

Thus, the second filter becomes redundant, hence the name “redundant filtration”.

Having multiple filters in the same tank might be completely benign, but sometimes it can cause some issues. You can still keep two filters simultaneously if you need to cycle a new filter.

But there are a few potential problems to keep in mind:

– Strong water flow

Excessive movement and turbulence are more likely in a smaller aquarium. Powerful flow happens when both filters are blasting at full GPH filtration capacity. The strong current can become stressful for the fish. It can also disturb free-floating plants and lightweight substrates like sand.

Luckily, this issue is easy to fix. Just remember the added flow rate of both filters should equal the ideal GPH for your tank size. The ideal flow rate for a 30-gallon tank, for example, is 200 gallons per hour. In general, one single filter should have a flow rate of four to six times the total volume of the aquarium.

– Excessive filtration

This ties in with the strong flow. If you’re trying to cycle a new biological filter, excessive water flow or filtration will hinder your efforts. The ceramic rings in the biological filter need time to accumulate nitrifying bacteria. The water that passes through the filter feeds them and helps them grow.

The longer the contact with the flowing water, the more time the porous material gets to feed new bacteria. If the water flows too quickly, the contact time between the water and the bacteria gets cut short. This means it will take longer for the bacteria to form, and thus for the filter to cycle. The filter will also not work at full capacity. Again, reducing the water flow can mitigate this issue.

– Filters underperforming

The performance will depend on the type of filters you have. If your new and old filters use multiple filtration processes, some will end up competing for the same water volume. This happens more with mechanical filtration.

The placement of the filters and the flow rate will also influence the performance of both filters.

You’ll have to keep an eye out for the mechanical filtration while the new filter cycles. Cleaning the tank more often during this period will help maintain water quality.

How Long to Run New Filter with Old Filter?

Nitrifying bacteria grow slowly. It will take a while before the new filter is fully cycled. Ideally, you should keep the two filters in the tank for at least three weeks. If the water flow is good, it should take about two weeks for the bacteria to start forming in the new filter media.

After that, you should give it an additional one to two weeks for the bacteria to proliferate and stabilize.

This is especially important if you’re cycling the new filter in a fish tank. About half of the bacteria in the tank is in the old filter biological media.

Removing the old filter too soon without fully cycling the new filter can be dangerous. A sharp reduction in the bacteria population will lead to quick shifts in water quality.

A sharp change in water pH or nitrate levels will shock or potentially kill your fish.

Why the Water Got Cloudy After Removing Old Filter?

You just removed the old filter from the tank. Suddenly, the water appears cloudy. This is more common than you might think, and not as dangerous as it seems.

The most likely cause is a bloom of heterotrophic bacteria. These bacteria are a natural part of your aquarium’s ecosystem.

Perhaps your new filter hasn’t fully cycled. You’ve also removed a huge chunk of nitrifying bacteria when taking out the old filter. This encouraged the proliferation of heterotrophic bacteria, which created a murky appearance.

Don’t panic! It should take a few days, or up to a week for the issue to settle.

As the nitrifying bacteria population kicks back into gear, the water will clear up. In the meantime, you’ll have to monitor the water quality for any shifts in ammonia or nitrites.

Performing small water changes and siphoning the substrate will keep the level of waste by-products stable.

You should also ensure the water is properly oxygenated. If you haven’t already, adding an air stone can help. Heterotrophic bacteria consume lots of oxygen.

Without proper water agitation, your aquarium’s oxygen reserve will deplete quickly. This can cause issues for the fish in the tank.

How do I Know if New Filter is Cycled?

You can’t see the bacteria in the filter media or the tank. But you should be able to see their effects. The easiest way to know if your new filter is cycled is by monitoring ammonia and nitrites in the tank.

You should perform a water test with both filters running first. After removing the old filter, wait 24 hours and perform another test.

The ideal reading for a cycled filter (and aquarium) should be 0 ammonia, 0 nitrites, and 5-10 ppm nitrates. Fish might tolerate higher values although those aren’t ideal.

Your fish can survive with <5 ppm nitrites and up to 20-40 ppm nitrates. There’s no upper tolerable level of ammonia.

Values above the ideal levels indicate that your new filter isn’t fully cycled. You shouldn’t add any more fish to the tank until the filter media has completed the nitrogen cycle.

Any additional bioload would push the ammonia and nitrite levels higher than ideal. In the meantime, you want to remove any excess waste and reduce the feeding quantity and frequency for your fish.

How Can I Speed Up New Filter Cycle?

It normally takes 4-6 weeks for a new bacterial colony to grow and establish itself. Cycling the new filter with an already established filter can speed things up. This method might cut the time down to 2-4 weeks.

If that still sounds like an awfully long time, there are a few additional things you can do to speed things up:

– Keep both filters running

You want the water to keep moving through the new filter as it cycles. This will provide more oxygen and nutrients for the nitrifying bacteria in the filter. Without enough oxygen, the bacteria won’t grow as quickly.

Sometimes, the growth might stall completely. So, avoid turning off the filters during the nitrogen cycle.

– Maintain an alkaline water pH

Nitrifying bacteria grow best in a pH above 7.0. Acidic water stalls or outright stops the proliferation of bacteria. So, you’ll want to bump up the water pH every time it becomes slightly acidic. Most fish tolerate a water pH between 6.5-7.8.

So, raising the water pH slightly above 7.0 shouldn’t pose any threats. You can bump up the alkalinity with small water changes or by using chemical buffers.

– Provide adequate temperature

Bacteria grow best under certain temperature settings. If the water is too cold, this will slow down the growth, sometimes by as much as half!

Luckily, most established fish aquariums should already provide the perfect temperature to support the nitrogen cycle. Temperatures between 65-85°F are ideal.

– Use concentrated nitrifying bacteria

Finally, what better way to get more bacteria than simply adding them yourself? Nowadays, you can find plenty of bottled concentrated bacteria for freshwater aquariums.

These products help speed up the cycling process by skipping the first bacterial growth phase. With the first bacterial strains instantly in place, all you have to do is wait for them to proliferate.

Note that these products can be hit-or-miss. Not all bottled bacteria contain the nitrifying strains you want in your filter. Also, these bacteria are highly sensitive to light exposure and temperature fluctuations.

Depending on the transportation and storage conditions, the little bugs might die before they reach your aquarium. But if you get a quality, live product, this will greatly speed up the cycling process.


Cycling a new biological filter in an established tank will speed up the growth of nitrifying bacteria. This will cut down the average duration of the nitrogen cycle from 4-6 weeks to 2-4 weeks.

You can also support the filter cycle by maintaining good water parameters and using concentrated nitrifying bacteria. Don’t forget that having two filters running simultaneously might cause some unwanted effects.

If you have a small aquarium, beware of excessive turbulence and filtration. You’ll have to lower the cumulative output of your filters to maintain appropriate flow for your fish and aquarium plants.

But remember to always keep the new filter running. You need oxygen and nutrient-rich water constantly passing through the biological media throughout the nitrogen cycle.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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