Do goldfish need a filter?

You should always include a filter in your goldfish tank or pond.

Goldfish eat a lot and produce a lot of waste, which can pollute their water. Without a filter, your aquarium water will soon poison your fish.

What type of filter do goldfish need?

There are two basic types of filter: external filters and internal filters. As the names suggest, external filters sit outside your tank, whereas internal filters are submerged within the tank.

There’s not a “correct” type and goldfish can be kept in tanks that use either an external or internal filter. However, an external filter is usually worth the extra cost if you can afford one.

As they sit outside the tank, an external filter can contain more filtration media and can therefore do a better job of cleaning your water. External filters can also be hidden away in an aquarium cabinet to avoid interfering with the look of your aquarium and have the advantage of not taking up valuable space inside the tank.

Wondering what size tank a Goldfish needs, check out our guide here

What size filter do you need for goldfish?

Black beard algae in Tank wit rocks and plants

Both external and internal filters are available in various sizes. However, it’s not the physical size of the filter that is important. It’s the “flow rate”.

Flow rate is the amount of water filtered per hour.

You should aim for an absolute minimum flow rate of 5 times the volume of your tank per hour.

Ideally, you should look to filter closer to 10 times the volume of your tank per hour.

For example, if you have a 20 gallon tank, your filtration system will be in the region of 100 to 200 gallons per hour.

It will be easier to reach a higher turnover if you use an external, rather than internal, filter.

Many types of filter – such as the Fluval U Series come in a range of sizes so that you can select the right filter for your tank.

Cleaning your filter

It is vitally important that you never clean your filter in tap water.

Instead, once or twice per year, you should rinse your filter sponges in water from the tank.

Take care to only rinse the sponges gently, rather than squeezing them too aggressively.

This is because cleaning your filter sponges in tap water, or squeezing them out too much, can kill the good bacteria living in the sponges. And killing this bacteria would be very bad for your fish!

Internal filters for fish tanks

Fluval U series filters

The Fluval U series filters are an excellent make of internal filter. They are one of the most commonly sold internal filters worldwide.

They come in multiple versions to fit a few different tank sizes and are great for beginners. They are simple to assemble, widely available and do the job of filtering most aquariums very well.

Fluval U1 Filter

We will start with the U1 filter, describing how it is useful, for what type of tank it should be used on and some pros and cons of the filter.

First off, the U1 filter is the smallest of the U series, being able to filter up to 55 litres. It consists of a typical internal filter container, with an impeller which has an adjustable flow rate.

The filtration contents are two reusable sponges, and a floss cartridge which needs replacing once every month or so.

This model is affordable, easy to clean and maintain and is versatile.

It is however, limited in size, as it can only power up to 55l. It is not an ideal filter for a goldfish tank as it is too small.

Pros and cons


  • Affordable
  • Small in size
  • Very easy to disassemble and clean
  • Easy to acquire parts for
  • Has an adjustable flow rate
  • Great for oxygenation and current
  • Impeller and pump are well designed
  • Blockages and breakages of the impeller are rare
  • Energy efficient


  • Has a small amount of biological filtration
  • Floss needs replacing every month
  • Top lid can snap if pushed too much
  • Gets clogged quickly if kept in a heavily stocked tank

Fluval U2 Filter

The next in line is the U2; this is perhaps the most commonly used out of the U series as it does up to 110 litres (30 gallons). 

Most people have anywhere from 10 to 30 gallon tanks, and so this is the best filter choice for the majority of setups.


  • Large filtration cartridges meaning the filter sponge doesn’t need to be cleaned as frequently
  • Has compartments for floss and carbon
  • Has a ceramic media compartment as well as a sponge compartment
  • Highly versatile filter can operate vertical or horizontal
  • Cartridges slide out easily and can even be replaced with different materials like cut sponge
  • Filter parts are reasonably priced


  • Is quite tall
  • Carbon and floss need replacing each month

Fluval U3 Filter

The Fluval U3 filter is almost the exact same design as the U2, but is taller and provides more biological filtration media, allowing it to filter up to 150 litres.

It has the same pros and cons as the U2 as it follows the same design, but is arguably much more suitable for a goldfish tank due to its increased filtration capacity.

Fluval U4 Filter

Again following the previous designs, the U4 is a taller version of the U2 and U3; it can filter up to 240 litres.

To summarise, the U series are excellent internal filters for beginners and seasoned aquarists alike. They are versatile, powerful and efficient filters which will keep your tank clean and fish healthy.

  • For goldfish, we recommend that you get at least the Fluval U2 as anything lower would not be powerful enough.

Even for goldfish in a 10 gallon tank, the U1 simply doesn’t have enough filter media to cope with the messy nature of the goldfish.

However, the other options are definitely adequate filtration units for goldfish tanks; just make sure you get the right filter for the literage and stocking of your aquarium.


Two orange Goldfish swimming in tank filled with green plants

The answer to the question “do goldfish need a filter?” is a very definite yes!

In order to take proper care of your goldfish, you need a filter of some kind.

This could either be an external filter or an internal filter, but we recommend using an external filter wherever possible.

However, it’s important to realize that installing any old filter is not enough to ensure clean water. It’s important to research the best filter for your goldfish tank.

You need to get a filter that turns over around 10x the volume of your tank per hour, make sure that you have a big enough tank, and always remove any excess food after feeding your goldfish.

Even with a filter, your water can quickly become toxic to your fish. This happens when your filter is too weak, your tank is too small, you leave food in the tank to rot, or you don’t do enough water changes.

3 thoughts on “Do goldfish need a filter?”

  1. I disagree that you need a filter. I’ve had goldfish for many years in both ponds and tanks without filters, and they are apparently doing fine since they won’t stop breeding and making babies that I have to give away. And I hardly change any water. Water plants take up the nitrates. The water in the pond can look pretty rank in the summer, but it’s always tested really well – no ammonia, nitrites or nitrates. For the pond, I make sure I don’t have too many fish, and I don’t feed them (there are enough bugs apparently). Goldfish are just carp, and they evolved in ponds that do not have filters.

    • Your comment is a particular scenario where your setup is self-sustainable, but is not the general case. Generally, any aquarium with live fish must have a filter, otherwise, the toxic waste will kill the fish in the tank.

      • Good comment Oscar,
        Some well established planted setups may be able to house fish without a filter, however, we always recommend a filter for general setups, especially to newcomers in the hobby.


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