Yes! Goldfish definitely need a filter!
You should always include a filter when keeping goldfish. Goldfish need a filter because they are very messy animals that eat a lot and produce a lot of waste (did you know that goldfish don’t have stomachs?). Unless you have a filter and do regular water changes, your aquarium water will very quickly become polluted and poisonous to your fish!
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.
Both external and internal filters are available in various sizes, which offer different “flow rates” (ie. different amounts of water filtered per hour). You should aim for an absolute minimum of 5x the volume of your tank per hour, and ideally closer to 10x 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.
So, the answer to the question “do goldfish need a filter” is a very definite yes! In order to take proper care of your goldfish and keep it healthy, you do 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. 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 if 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.
Finally, it is vitally important that you NEVER clean your filter in tap water. Once or twice per year, you should rinse your filter sponges in water from the tank, taking 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!