Goldfish filtration and Media

Goldfish are very messy fish – they seem to make more waste, both from their gut and also from their gills than another fish of an equivalent size – this means that they definitely require a filter to detoxify the water, but there are various types of filter and media to consider…

It is important that your goldfish tank is big enough for the number of goldfish you have – most people here recommend a minimum of 20 US Gallons for the first fish, and 15 to 20 more Gallons for each extra fish.

Remember that although goldfish might have been bred for life indoors, they did evolve as fish which lived in massive bodies of water, where there were bugs to deal with the tiny amounts of dangerous toxins….. And though they were captivally bred for life in ponds, even a pond is much larger than a tank, of any description.

Most people start with a tank which is too small for their fish – I did – I started with a tiny tank which the pet shop advised would be suitable for two fish, but was way too small for even one!

But just as important is a filter – the filter (the thing that pumps water and) contains media which is the stuff where the bugs will eventually colonise and live, when the tank is cycled.

There are many types of filter and many types of media to put in them.

Basic rules for a filter for a goldfish tank are that it should be able to turn over the water in the tank ten times per hour – so if you have a 50 Gallon tank, you will need a 500 Gallon per hour filter. Likewise a 200 litre tank will require a 2000 LPH filter.

These “rules”, suggestions, will not allow you to safely exceed suggested stocking levels – having a 2000 LPH filter on a 50 litre tank will not allow you more fish than should fit it a 50 Litre tank. The Goldfish do also need room to grow…. They would also perhaps be blown about by such a huge water flow in such a small tank? Perhaps not, but no way four times as many fish will work… For very long.

“But they are just Tiddlers”! How can these tiny fish need that much space!!!!?

Perhaps right now they may not, but not for long – if they are to survive and thrive.

There are many types of filters which can be used in a goldfish tank, but most keepers use either an internal filter, a Hang On Back (HOB) filter or an external canister filter.

Each type of filter has its advantages and disadvantages.

Internal filters are probably the type that most fish keepers would least recommend, mainly because they take up space inside the tank – they reduce the water volume, whereas a filter outside the tank will increase the total water volume, perhaps not by a huge amount, but also internal filters tend to be too small to adequately filter the tank for goldfish. They also don’t look as good as a filter which could be hidden a bit.

With any type of filter, be aware that if it says it is suitable for a tank of up to XX Gallons, that might be true in certain circumstances, but unlikely for messy goldfish – it is important to find the actual flow rate – do your research before purchasing!

HOB filters seem to be the most commonly used type of filter, in North America at least – I live in Europe and have never seen one for sale anywhere! They are available in various sizes, suitable even for larger tanks. If the tank is too big for one large one, you can use two of them to double the capacity.

HOBS often have a larger internal volume for media than internal filters, but many are still supplied with replaceable cartridge filters – a plastic blade which fits into the filter and contains mesh, some biomedia and often some activated carbon. It is recommended by the manufacturers that these be replaced regularly – activated carbon has a limited lifespan. If your HOB has two cartridges, you could stagger the change schedule, perhaps one every couple of weeks, but if it only has one, most likely you will be throwing most of your bacteria away with that cartridge, possibly causing a “mini cycle” in the tank, or worse. HOBs can be populated with other types of filter media which can be better than the manufacturer supplied one and can last longer, perhaps indefinitely, saving money and avoiding cycle bumps.

External canister filters generally work out to be the most expensive type of filter, being much larger than HOBs. But being larger they have a greater volume than HOBs and often come with baskets which can be populated with your choice of media. Because of the increased volume of media contained, it is possible that an external canister filter with a turnover as low as 5 times may be suitable, even for goldfish.

Canister filters take up more space than HOB filters, but HOB filters prevent your tank going as close to the wall behind it if there is one – they sit behind the tank. External filters need to be sited below the tank, but can often be placed in an aquarium stand, and hidden away from sight.

As there are hoses connecting the outflow to the canister filter and a return hose pumping the water back to the tank, any leak could cause a significant flood issue. Leaks on the outflow side will unlikely cause a flood as the leak will suck air rather than leak water, unless there is a power cut, but a leak on the return pumped side could empty most of the tank water onto the floor. That is a risk, but hose checks during filter maintenance should minimize that risk.

A blocked HOB filter could also flood, but perhaps at a slower rate than an external filter.

One disadvantage I have recently read about with HOB filters is that they seem to be more prone to impeller block ups, than an external. An external canister filter is gravity fed – dirty water is filtered before then cleaner water is pumped back up to the tank. Conversely, a HOB pumps dirty water up to the chamber where filtering happens, and then gravity trickles the water back in.

There are many other types of filter available – such as a sump – an extra tank under the tank, which could even house plants and a huge amount of media.

Undergravel filters use air pumps to flow water through the gravel in the tank – effectively making the gravel your filter media. These seem to have fallen out of fashion, because although they are simple, many here would not recommend gravel, as your Goldfish could swallow it, causing a blockage. Also I have read that in a power cut, an undergravel filter could quickly release a significant amount of toxins back into the tank water. They are cheap and simple, but probably not suitable for messy goldfish.

Some aquariums come with filters built into the hood of the tank, and DIY fishkeepers sometimes make wet/dry or trickle tower filters.

Most commercially available filters will be either internal, HOB or external canister.

Members here all have their personal favorite brands and types of filter – here are some comments;

Aqueon – less expensive, room for some additional media but then there’s no mechanical filter before the added media, changing the cartridge removes a substantial amount of beneficial bacteria. I saw on YouTube a way to “Turbocharge” this filter, by adding a foam over the intake strainer. Intake strainer foams seem to easily clog, which can reduce flow rates, even to the point that there is no water reaching the impeller, which will likely cause overheating and serious issues. Another member mentioned a burned out motor – they are water cooled!

Some members claim that the impeller can be clogged by small particles, others claim that they have never has such an issue…

AquaClear HOB – maintenance is easier, more space for more media, never jams, maybe noisier. They seem to be more expensive.

MarineLand – there are just better options for better prices. They aren’t always as well made and don’t last as well as the Aqueon or Aquaclears. The biowheels don’t really hold enough Good Bacteria to keep up with gross messy goldfish.

Tetra/Top Fin – Poor quality materials, not very powerful, break down or become very noisy. Tiny Whisper HOB’s are good as are their air pumps.

Seachem – a respected manufacturer has recently released a HOB which seems to get good reviews, but no one I know has used one yet, but they are a reputable company – it might be worth looking into their product?

Sun Sun canister – absolutely silent, good power, very inexpensive, but doesn’t come with much media so it’s up to you to fill it, and assembly instructions are terribly translated.

In the UK, the Sun Sun brand is available branded as an All Pond Solutions filter – it is supplied with media – carbon, ceramic hoops and foam. I would personally rather own an Eheim or a Fluval canister filter – some of their models can self regulate flow rate and advise when it is too low – that it requires maintenance, but really an external canister is just a pump and a space for media, so the most important aspect is volume and flow rate, and maintenance!

Before deciding what to purchase, you will have to do some research, and maybe ask some questions here.

One more thing – an external canister filter dumps the water to the bottom of the external filter – the water is filtered as it rises back up through the media. Many people think that the biological protecting foams should go at the top, or in the top basket, but for most filters, this is the wrong way round. Foams are there to prevent the biological media from clogging, so the water must hit them first. Foams first (in the bottom tray), as much biological media  as you can fit or afford next, and perhaps leave a space in the upper tray, should you need to add some specific items?

Filter media.

There are three categories of filter media – Mechanical, Biological and Chemical, though there can be significant crossover between the types, and not all are necessary at all times.

Mechanical Filtration is often accomplished by sponges – a course, medium and fine foam or any other combination, will strain debris from the water. This is necessary to protect the biological media to try to prevent it getting clogged up, but foam itself can also act as the biological media, as the pockets in the sponges can house the bacteria we need to convert ammonia and nitrites.

Biological media is an inert substance solely there to allow colonization by the benefical bacteria. Lava rock, ceramic “noodles”, bioballs and even pot scrubbers can be used to house the bugs. The general rule of thumb is that the greater the surface area of the media, the more bacteria it will be able to accommodate. But therefore porous substances with microscopic holes need to be protected from getting gummed up and clogged by dirty water – they will last longer if adequately protected by a mechanical filtration first.

Chemical filtration may often not be necessary in an established tank – I have personally never used any, but activated carbon can be useful to remove medication from the water, and zeolite can be used to help remove ammonia in an emergency – it absorbs the ammonia. Zeolite could be used in the filter, but also just placed into the tank in a filter sock (mesh bag). It can be removed and recharged by leaving it soaking in 5 percent aquarium safe salt water. It can then be dried and reused, but not indefinitely.

Obviously Activated Carbon and Zeolite should not be essential in a healthy stable tank, but can be useful in certain circumstances, but a water change would accomplish much the same, with less expense.

Many thanks to – in no particular order – Tanki, Shawn, Myra, Ginger, Kass, and Blue, for their help and input.