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Behaviour Guide - Goldfish

Updated: Apr 10

Cats purr, dogs wag their tails. Some animals can even smile. But not fish - they’re a little more difficult to read. As a pet owner you care about the little lives that depend on you, so being concerned over the happiness of your fish is quite common. Being able to speak to your pet would be amazing - knowing when your dog needs to go outside or your cat needs to go to the vet just by asking them.



Unfortunately, our pets don’t speak. But that doesn’t mean we can’t begin to read their behaviour and body language to make an informed guess on how they might be feeling.

Through this article we’ll be homing in on the humble goldfish, explaining some of their behaviours and trying to figure out what they mean so that we can understand their needs and wants.


When goldfish chase each other.

Goldfish chasing each other can mean a couple of things. The first meaning potentially is that they are being quite friendly with each other - and trying to mate. If this is what the behaviour is then you’ve got nothing to worry about as it isn’t aggression or bullying that’s going on. You might find some little blobs one day though - fish eggs. If you want to raise the new eggs then there’s a whole process you’ll have to follow to keep them safe as the adult fish may try to eat the eggs, they may float up into the filter. Basically they need to be relocated and kept safe.


The second option for chasing though, is aggression. This is quite rare in goldfish however, as they’re pretty docile and don’t like fighting with other fish. However, stress can change anyone. If they aren’t being fed enough or they’re overcrowded or bored then they need to be seperated and the underlying cause needs to be addressed. If this doesn’t happen, the cause isn’t corrected, then your fish can become ill and die.


Related article: Goldfish Care Guide


Goldfish sitting at the bottom of their tank

This could be another simple matter and one you don’t have to intervene with, or it could be something else. Here’s an example: Your goldfish has been swimming around and active for most of the day but has slowed down and is just sat at the bottom of the tank. It’s likely that they’re just taking a break or having a nap.


However if you’ve noticed that your fish is at the bottom of the tank and has been for a prolonged period - a few days perhaps, then you may have a sick goldfish. Usually this means that your tank hasn’t been cleaned recently in terms of a water change, or perhaps the filtration equipment may have failed.


The filtration system should have the proper mechanisms to maintain the chemical hazards in the tank water, the biological (algae) hazards, as well as the mechanical hazards (physical debris) that could be floating in the water.


If the filtration system does not maintain all three aspects of water filtration or, if the device is simply not working properly then, this issue will need to be corrected. If, however, all seems well with your filter then perhaps consider a 30-40% water change or cleaning your aquarium and see if this changes anything. By doing this you’re diluting a potential build up of pollutants, be sure to grab a water testing kit from your local pet store and check nitrites, nitrates, ammonia and pH levels of the water. By changing the water you’re also adding fresh oxygen to the tank.


Did you make sure to cycle the tank before adding your goldfish? Find out how your tank should be prepared long before you add your fish here.


You can also find our recommended filter for under 100L tanks here, and our recommended filter for over 200L here.


Goldfish bobbing around at the surface of the tank.

Having a goldfish at the top of the tank is usually indicative of one thing - a dying fish. Goldfish tend to explore the whole of the tank, so if you have one that’s only bobbing up to the surface then they may be sick, or the water may be low in oxygen. Goldfish low on oxygen tend to gasp at the surface of the water to try and breathe. If this is the issue then you need to perform a water swap on around 50% of the water, the new water will have more oxygen in it and the splashing of the water being added will also add more. If your tank is clean and this is happening then look into buying an air pump and perhaps some live plants to convert CO2 into oxygen.


Overcrowding, missing out on water tests, a broken filter etc can all be things that combine into oxygen level issues. If you don't have an air pump either, you can pick one up here.


Foraging fish

Sometimes goldfish might dig up their plants or push small ornaments around - this is known as goldfish foraging. This is typically normal behaviour, goldfish like to snack between their meals. If you don’t provide them with treats then they’re likely to hunt for them around the tank instead. Treats could be things like nibbling on plants, digging up plants to help them munch on them and the roots or they might even have a nibble on algae.


This is a normal behaviour for goldfish, but if they’re doing this immediately after eating, or they aren’t eating their actual food then this is a cause for concern. If they’re leaving their food it might be that the food isn’t nice, try buying them different food. If they’re eating their food then foraging, try feeding them some more.


Fish process food quite quickly which can leave them hungry throughout the day, but this isn’t a reason to feed them excessively - they should be fed a small amount that they can consume within 3-5 minutes, twice a day. If you have trouble judging how much they need to eat, or your busy schedule makes it difficult to feed them at set times then consider buying an automatic fish feeder.


A nice treat for Goldfish is actually Brine Shrimp, you can buy them freeze dried here, or check out our article on how to set up a brine shrimp hatchery, so that you can give live food.


Aggression in goldfish

Goldfish usually aren’t aggressive as we’ve discussed. They’re usually pretty docile to all life forms however, they can become aggressive if something is not right. Sickness and disease can lead to stress, as can inadequate living conditions, be that poor water quality or overpopulation.


Typically there are ways you can treat this. Look at the situation and try to work out what could be wrong. If there’s adequate space, then test the water. Look at the fish to see if there’s any changes in colour or marks. Check how often you’re feeding them. This should help you work out if the fish is suffering from disease or illness.


How much space is there in the tank? It should have a decent amount of space for the fish to swim around. Is the tank overcrowded? As a rule of thumb there should be around 30 gallons of water per fish in the tank. So your tank may be large, but if you have 10 fish then this might not be enough room.


If you can’t get a larger tank and you need one, or you haven’t got space for a single large tank, consider splitting the fish into smaller tanks instead.


If parasites in the tank have made your goldfish ill and aggressive, then you will need to treat the aquarium water with a parasite medication that will kill them and stop their eggs from hatching and continuing the cycle.


Sick goldfish should be removed from the tank and quarantined in a separate tank if they are the only unwell fish.


Rushing to the top of the tank.

This is another behaviour that could result from multiple explanations. The first point could be that they’re rushing to the top of the tank to grab oxygen, suggesting there isn’t enough in the tank. To fix this, perform a quick fix of a 30% water replacement and then invest in an air pump to maintain good oxygen levels. You can also add plants and these will capture CO2, replacing it with oxygen but at a slower rate than the air pump. If you have a large tank you may be able to add multiple plants instead of an air pump, if you’d like to save on energy costs.


Investing in a larger tank if yours is quite small will also benefit oxygen levels, as more water means more oxygen can be stored. You’ll also want to check the water for excessive ammonia, nitrite and nitrate levels.


The other reason is much less panic inducing - it’s simply close to feeding time. Goldfish are a lot more intelligent than we give them credit for. Many people think that goldfish have a very low memory span, explaining why they are hungry constantly. In fact, this has been disproven by scientists teaching goldfish tricks and getting them to memorize mazes. Truth is, goldfish just don’t know when they're full. They do however, have a sense of time. And they can learn to judge when feeding time should be.


Your goldfish may just be swimming up to the tank to greet you, as they are capable of remembering human faces, as well as greeting any treats or food you may have!

If your fish doesn’t look like they’re in distress, gasping or struggling in any way then it may just suggest that they’re actually just happy to see you.


In conclusion, goldfish are fascinating and mysterious creatures. They are intelligent, delicate, yet hardy and strong beings that require our constant care. You can often seem them playing, chasing, feeding, foraging, napping, and swimming together as friends in a group. These behaviors typically represent a happy and healthy goldfish tank.


However, behaviors such as lethargy, stillness, swimming upside down, scratching against objects in the tank, or gasping for air at the water’s surface typically demonstrate a goldfish that is suffering from an illness or disease. Observation can mean the difference between life and death for your goldfish.


You can also read up on common care mistakes new owners tend to make, or if we haven't answered your question here, you can also read up on the most common fish illnesses.



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