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Thoughts on Filtration

Filtration is probably the most important thing any fish keeper should be thinking about, but like so many important things in life, it’s hard to get excited about your filter. You’d rather think about the gorgeous fish you love, the aquascaping, the coral displays, the pond design, or almost anything but the boring old filter.

The short video above illustrates a filter upgrade performed by one of our dealers resulting in an easy to maintain filter and a beautiful, healthy tank.

Filters are generally not that attractive. They can be pretty expensive and you have to clean them periodically, which is never a pleasant experience. But, the fact remains, they are the most important component in your system and the health and therefore the appearance and activity of your livestock depend on how well they perform.

Filter Functions

Any tank or pond filter has to perform two functions. Ideally, these will be handled separately as cleaning and maintenance requirements are different for each function.

First, the filter must perform mechanical filtration to remove visible debris from the water stream to maintain the clear water most of us desire so we can appreciate the beauty of our aquatic friends. And, a lot of visible debris contains chemical components or support organisms we don’t really want moving around in the same water as our pets.

Mechanical filters range from Rotating Drum Filters with automated back-flush, Brushes, Fiber Mats to Vortex Settlement Chambers in ponds. In aquariums Reticulated Foam, Fiber Mats, Socks and almost always some sort of Settlement Chamber. Regardless of the type since they are dealing with things you can actually see, they must be cleaned from time to time which is always more or less a messy business.

Once the physical debris is taken care of, the toxic chemical waste must be dealt with. Mother nature takes care of this by providing several types of bacteria that consume these chemicals and convert them into other, less lethal forms. All the keeper has to do is provide suitable media in a chamber of adequate size and let mother nature do her thing.

Filter Sizing

Filter manufacturers always indicate filter size based on the tank or pond size. This is logical since the customer will know the size of the aquarium or pond they need to filter, but there are a couple of things you need to keep in mind.

First, this is based on a reasonable fish load and feeding plan. So if your self-control is not the best and you just can’t pass up that fantastic finned beauty you just saw and your fish load is way more than recommended, you’ll need a larger filter.

If you’re a competitor showing your koi and determined to raise a grand champion, you’ll need to maintain a much smaller fish load than the casual hobbyist. You’ll be feeding several times as much and you’ll need a much larger filter.

A common rule of thumb is to get a filter for a pond or tank twice as large as what you have. If you’re planning on using Biohome® media for your biological filtration to reduce nitrates we provide more detailed rules of thumb to determine the amount of media you need which will allow you to determine the size of filter required.

Filter Types

It appears the most popular filter types for koi keepers these days are Rotating Drum Filters (RDF) for mechanical filtration combined with a Shower for biological filtration. The RDF provides very clear water with minimal maintenance and the Shower can easily be sized large enough for whatever media you select. The shower provides good aeration as the water falls through the trays.

Other types of pond filters include Bead Filters, Multi-Chamber Filters and for smaller ponds all sorts of configurations.
For aquariums, we have Hang on the Back (HOB), Canisters, All in One, Sumps and DIY filters. It’s possible there are others, but that covers most of the ground.

HOB filters are a good choice for smaller aquariums, although most of them depend on chemical, that is Carbon, filtration with limited to no space for your choice of biological media. Carbon filters work by absorption and therefore will eventually fill up and so require periodic replacement. This can get expensive.

There are only a couple of manufacturers selling HOB filters that are designed to allow you to use your choice of media. They feature simple designs and would be our suggestion if your tank is small enough. These are the Fluval AquaClear series, AquaClear 20 – 110. The numbers indicate the maximum size of the tank in gallons the filters are designed for. We’d recommend you use a 110 on a tank no larger than 15 gallons, which gives you an idea of how overrated filters are when considering nitrate reduction.

A similar HOB is the Seachem Tidal 35 – 110 series. The Tidal 110 would be suitable for tanks up to 30 gallons if filled with Biohome media.

There is an almost infinite number of Canister filters available in a variety of sizes. You can almost certainly find one or more, it sometimes takes more than one to provide adequate capacity, suitable for your tank. The Pond Guru, a distributor of Biohome media in the UK, provides a useful service for UK hobbyists. He accepts filters that he hasn’t seen before from hobbyists, rebuilds them and posts videos showing the proper way to configure them on YouTube.

You can find him here: His advice can be very helpful for selecting a suitable filter for your aquarium. We have a much smaller collection of upgrade information on our How Much Do I Need page.

A number of smaller tanks now come with a section for a Built-In Filter. These are similar to a Sump but built right into the rear or end of the aquarium. They usually have three sections. An inlet section where you can install your mechanical filtration. A center section for biological media and the outlet section where a return pump is located and you can install a heater if necessary.

This is a pretty convenient solution. They normally provide enough space for adequate biological media for the tank size and are reasonably easy to maintain.

Larger aquariums and more sophisticated users will often use a Sump filter. This is essentially another aquarium divided into three or more sections mounted under the main tank. Some configurations don’t provide means to effectively provide for mechanical filtration. They often depend on Sock filters which provide fine filtration, which is extremely effective but plugs up pretty rapidly. For this reason, we always recommend using Coarse, Medium and then Fine mechanical filtration to provide a reasonable period between cleaning. So if you’re looking for a sump check out the mechanical filter section carefully so see if it will accommodate this and how easy it will be to maintain.

Sumps are available in a large number of configurations and can be made to order so will accommodate whatever sort of filtration you desire or need with lots of room for refugiums, reactors, heaters and whatever else you might want to house there.

And finally, there are DIY filters, whether for cost concerns or just the way they’re wired some people will always rather build their own. Nothing wrong with that as long as they provide easy to maintain mechanical and biological filtration.

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