Koi fish can be real messy. It’s why having a reliable filter is so important.
Working to continuously clean your pond’s water, it helps ensure the chemistry remains balanced and remains a healthy environment.
Choosing the right filter for your koi pond can be a tricky process. To help, I’ll walk you through everything you need to know about finding the perfect filter for your pond setup.
Plus, I’ll review five of the best koi pond filters on the market today to make getting a new filter in your pond fast and painless.
Buyer's Guide: How to Choose a Filter to Fit Your Koi Pond
Not all koi pond filters are built the same, and different filter systems will work better or worse for different styles of ponds.
Here, I’ll take a closer look at some of the most important features to consider when choosing a koi pond filter so you can determine what will work best for you and your fish.
Every koi pond filter has two stages – biological filtration and mechanical filtration. These two stages operate very differently, but both are essential to ensuring the health of your koi.
Biological filtration involves transforming biological waste like ammonia to maintain the water chemistry of your pond as well as eliminating unwanted microbes from your pond.
Biological filtration stages typically use a media substrate that encourages the growth of beneficial microorganisms on the filter.
These microorganisms in turn convert ammonia, which your koi produce as waste, into nitrates that can then be utilized by the plants living in your pond.
Often, having a biological filter with beneficial bacteria will also help to colonize the rocks and substrates in your pond with these ammonia-transforming microbes.
Another, separate stage of biological filtration uses ultraviolet (UV) light to kill off unwanted microbes, such as parasites and bacteria that can spread disease among your koi.
When your filtration system includes a UV light, keep in mind that the effectiveness of this light requires that your pond water isn’t too silty or passing through the filtration system too quickly – the longer the water is exposed to the UV light, the more effective the treatment is.
So, getting your pump flow rate will be extra important when using a UV light.
Mechanical filtration is simpler than biological filtration, but just as important. Mechanical filtration uses a physical filter to sieve out particles, such as large bits of food waste and algae, from the water.
Ideally, your filtration system will have multiple layers of mechanical filters in a range of sizes so that it can collect the widest selection of particles without clogging.
How Much Filtration Do You Need?
The amount of filtration you need for your koi pond depends on two things: the size of your pond and the number of fish in it.
Filters are rated for the volume of water in your pond, so that’s the best place to start. However, a pond with koi fish in it will require more filtration capacity than an empty pond because of the waste and crumbs these fish produce.
As a general rule of thumb, you should get a filtration system that offers two to four times as much filtration capacity as the volume of your pond.
So, if you have a koi pond that is 1,000 gallons, you should get a filter that is rated for at least 2,000 gallons. If your pond has a large number of koi or is in direct sunlight for most of the day, aim for a filter rated for 4,000 gallons for that same 1,000-gallon pond.
Keep in mind that this calculation provides a minimum filter capacity – when in doubt or if you are between filter sizes, it is always better to opt for having more filtration capacity than you need.
Pressurized or Non-pressurized Filters
Another choice you’ll need to make is whether a pressurized or non-pressurized filter is better for your koi pond. There are advantages and disadvantages to both systems.
Pressurized Koi Filters (Bead Filters)
Pressurized koi filters, also known as bead filters, are very popular for koi ponds because they offer the most flexibility in designing your pond. Pressurized filters can be installed at or below ground level and are capable of processing huge volumes of water.
Water flow through pressurized koi filters is significantly slower than through non-pressurized filters, which has several important consequences.
This slow water movement leads to better filtration overall since the filtration media have more time to filter the water moving through the system.
On the other hand, beneficial bacteria don’t thrive under slow-moving water conditions because this water can be more depleted of the oxygen they need to process ammonia.
Still, this oxygen depletion is primarily a problem only in ponds with a large number of koi. Even then, you can give your biological filtration the boost it needs by adding a pre-filter or air bubbler into your filtration system, or by adding activated carbon to your biological filtration media.
· Can be installed above or below ground
· Capable of filtering very large water volumes
· Slow water movement allows more time for filtration to work
· Leads to oxygen depletion, which can negatively impact biological filtration
Non-pressurized (Box) Koi Filters
Non-pressurized filters, also known as box filters, are simpler and easier to install than bead filters. However, these submersible filters do place some limits on the design of your pond since they have depth restrictions and require a flat bench to sit on.
The advantage to these filters over bead filters is that they don’t reduce the flow rate of water coming from the filter pump. That means that beneficial bacteria growing on the filter media have plenty of oxygen and can work very effectively in crowded koi ponds.
Better yet, the higher flow rates of non-pressurized filtration systems allow biological media to clean itself, as dead bacteria will be knocked off by the flowing water. At the same time, there are no negative effects of the higher flow rate on mechanical filtration.
While not necessary in most ponds, it’s also possible to use a non-pressurized filter as a pre-filter in front of a bead filter. That way, you get the benefits of highly effective biological filtration along with slow, high-volume mechanical filtration in the bead filter.
· Easy to install
· Excellent biological filtration
· More affordable than bead filters
· Biological media is self-cleaning
· Places limitation on pond design
5 Best Filters for Koi Ponds Reviewed
Now you have a better understanding on your options, here are my choices for the five best filters for koi ponds.
1. Oase BioSmart Koi Filter
This non-pressurized filter offers superior biological filtration for ponds with a large number of koi.
The filter includes multiple biological media filters, each boasting a huge amount of surface area. Plus, the filters are split into multiple zones to better facilitate the biological conversion of ammonia to nitrite and then to nitrate.
The filtration system is straightforward to clean thanks to a built-in cleaning indicator and sludge drain. To clean the filter, all you have to do is turn off the pump and pull up on the sludge drain handles.
The downside to this filter is that compared to other non-pressurized filtration systems, installation isn’t all that straightforward because of poor instructions. In addition, there is no UV filter built into this system to remove harmful bacteria.
· Biological filtration split into multiple zones
· Easy cleaning thanks to sludge drain
· Available in 5,000 and 10,000-gallon sizes
· Poor installation instructions
· No UV filter
2. Oase BioTec Screenmatic Koi Filter
This massive non-pressurized filter system from Oase is essentially the larger brother of the BioSmart filter.
It features the same multi-layered biological filtration system, albeit with a new and improved foam design around the filters to ensure no water leaks out of the filter box. It also has a sludge box, which has been enlarged to meet the significantly higher capacity of this filter.
One new feature is a self-cleaning module, which allows you to alter the water flow through the system to separate debris from the filters. In addition, water level optimization goes a long way towards minimizing clogs.
Unfortunately, Oase didn’t do anything to address the main shortcoming of the BioSmart filter in this new and larger model. The filter still doesn’t come with an integrated UV light, although an add-on clarifier is available.
· Large filtration capacity
· Enlarged sludge box
· Multi-layered biological filtration
· Water level optimization
· No UV filtration
3. Goplus Pressure Bio Filter (Budget Friendly Option)
This small pressurized koi pond filter from Goplus offers excellent filtration for a wide variety of pond systems.
The inclusion of a 13-watt UV clarifier in this filter is a major plus since it takes advantage of the slow water flow to eliminate harmful bacteria and parasites. Cleaning is straightforward thanks to the simple design of the canister housing, which unscrews and provides access to all internal components.
However, as with many pressurized filters, water oxygenation is an issue to be aware of. If your pond has a large number of koi, you may need to add activated charcoal to the filter media that comes standard with this system.
While this pump is effective and inexpensive, be wary of the build quality. The filter housing appears to have manufacturing issues and many units have one or more leaks.
GoPlus’s customer support has been severely lacking in responding to issues with the filter system and the company does not offer a warranty.
· Includes UV filter
· Easy to clean and maintain
· Good for small ponds
· Poor oxygenation in crowded ponds
· Leaky filter housing
· No warranty and poor customer support
4. Bubble Bead Filter XS-1000
This bead filter from AST offers the easiest cleaning of any filtration system we’ve seen.
To clean it out, all that’s needed is a quick backwash – typically, this is enough to reduce pressure build-up in the system without having to remove or replace any of the internal filters.
Thankfully, this backwash is still gentle enough to avoid breaking up the biofilm on the biological filtration media.
Like other bead filters, this unit specializes in mechanical filtration. AST claims that the unit can capture 100% of particles greater than 50 microns and fully half of all particles down to 5 microns – ensuring that your water will remain clear.
However, keep in mind that this filter doesn’t come with a UV light to kill harmful bacteria.
The filter is available in sizes ranging from 300 to 20,000 gallons. While AST doesn’t offer a warranty, you can expect five-star customer service from the company.
· Simple backwash cleaning
· Excellent mechanical filtration
· Wide range of filter capacities
· Very good customer service
· No UV filtration
· No warranty
5. Aqua Ultraviolet Ultima II
Aquarists rave about this bead filtration system from Aqua Ultraviolet. It effectively solves the problem of poor biological filtration in bead filters by re-designing the biological media filter.
In this system, you’ll find a ridged substrate that offers the most surface area for beneficial bacteria and which has been proven in tests to keep up with ammonia transformation in non-pressurized filtration systems.
Better yet, the Ultima II is simple to clean with a gentle backwash. To initiate the backwash, all you need to do is turn a valve on the side of the unit – there’s no need to open it up or interrupt your filtration for longer than a few minutes.
Although Aqua Ultraviolet’s brand name suggests that this filtration system comes with a UV light, that’s not the case. You can add on a UV light in-line to the filter, but this is not a standard feature.
· Highly effective biological filtration
· Simple backwash cleaning
· Relatively inexpensive for a bead filter
· One-year warranty
· No UV filtration
Which Koi Pond Filter is Best?
The best filter for your koi pond ultimately comes down to your specific needs. However, the filtration systems we reviewed offer advantages and disadvantages for many different types of ponds.
For example, the Aqua Ultraviolet bead filter is a good choice for aquarists who want a bead filter that doesn’t compromise on biological filtration – but also doesn’t have a koi pond that exceeds the limits of a 6,000-gallon filter.
The Bubble Bead and Oase BioTec Screenmatic filters offer options for larger koi ponds, and both feature simple cleaning mechanisms. Meanwhile, for aquarists on a budget, the Goplus pressurized filtration system offers a budget-friendly solution for small koi ponds.
Overall, the my top pick for the best koi pond filter would be the OASE BioSmart 10000.
Koi Pond Filters FAQ's
Should I add a pre-filter?
The majority of koi ponds and filtration system setups won’t require a pre-filter – the mechanical and biological filtration in your system should already be good enough to keep your koi healthy.
But, if your pond is heavily stocked with koi, you may notice that the filter isn’t catching everything and debris is building up. In this case, adding a mechanical pre-filter can go a long way towards keeping your pond clean.
Alternatively, adding a non-pressurized filtration system as a pre-filter in front of a bead filter can be a good idea if you’re worried about inadequate biological filtration. Again, this is only necessary in the most heavily stocked koi ponds.
How often should I clean my filter?
There’s no set time-frame on how often you should clean your filter since it varies a lot between filter systems and ponds. The best way to know when to clean your filter is to keep an eye on the outflow from the system.
When you notice the outflow start to slow down or lose pressure, it’s time to clean your filter.
If you’re worried about cleaning, consider choosing a high-end filter like the ones from Oase that offer a built-in cleaning indicator.
Or, simply backflowing a pressurized filter from time to time can help ensure that you never run past the necessary cleaning timeframe.
What size of pump do I need with my koi filter?
Your pump should be large enough to filter all of the water in your koi pond every hour. So, if you have a 5,000-gallon koi pond, you should use a 5,000-gallon pump or slightly larger.
However, if you’re using a bead filter it’s important to keep in mind that the flow rate of your pump will affect the pressure in the filtration system.
Read the manual for your bead filter to ensure that you don’t overpressurize and blow out your filter housing by using too powerful a pump.
Getting the right filtration system is one of the most important aspects of establishing and maintaining a koi pond.
With the best koi pond filter, you can ensure that your pond water stays clean, the chemistry remains balanced, and your koi live a happy and healthy life.
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Last update on 2019-08-22 / Affiliate links / Images from Amazon Product Advertising API