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Home > Goldfish > 9 Mistakes to Avoid Making as a Goldfish Keeper: Vet-Reviewed Errors & Prevention Tips

9 Mistakes to Avoid Making as a Goldfish Keeper: Vet-Reviewed Errors & Prevention Tips

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Dr. Luqman Javed

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We all want to be the best goldfish keepers for our aquatic friends, and we care a great deal about our goldfish and providing the happiest, healthiest lives for them. While there are people who choose to remain willfully ignorant of proper goldfish husbandry, most owners genuinely care about their pets. Of course, everyone can unintentionally make a mistake, especially one that is extremely common among new and inexperienced goldfish keepers.

If you are dealing with a sick fish or are attempting to remedy a mistake that you made, it can be easy to berate yourself, but there’s no need to! In this article, we’ll cover some common mistakes that new goldfish keepers might experience. So, read on!

divider-fish The 9 Common Goldfish Keeping Mistakes

1. Not Cycling the Tank

This is easily the most common mistake that people make when it comes to keeping goldfish or any other fish, for that matter. Most people are used to the simplicity of going to the store, buying a tank and a fish, and taking it all home to get started. However, this doesn’t allow for proper tank cycling.

A tank cycle is the process of establishing beneficial bacteria colonies within the tank. These colonies live in the filter, substrate, and multiple other surfaces within the tank where water flows. Beneficial bacteria break down ammonia, which is a naturally produced metabolic waste product from fish and decomposing organic matter and convert it to nitrite, and eventually nitrate.

In an aquarium setting, nitrate is the end product of the nitrogen cycle and is the main reason that you must perform water changes in a fish tank. Plants will also help reduce nitrate levels in the tank, using these as a fertilizer for growth.

It is possible to perform a fish-in cycle, which means you keep the fish in the tank as you are cycling it. However, this is not ideal and is no longer considered humane in modern-day fishkeeping. The main part of performing a tank cycle is to enable ammonia levels to become such that the beneficial bacteria have something to consume for energy, growth, and reproduction.

Ammonia and nitrite can both be damaging to fish, temporarily or permanently, and the ideal ammonia and nitrite levels for a tank with fish living in it are zero. As you can imagine, this makes it difficult to safely perform a fish-in cycle. There are products available that contain beneficial bacteria, which can help jumpstart your tank cycling. These are not adequate replacements for performing a tank cycle, though.

Adding fish into an aquarium that isn’t cycled runs the risk of New Tank Syndrome1. Goldfish, being very high waste producers, are very susceptible to this condition. Therefore, it is important to cycle your tank (using a fish-less method) before adding your fish in.

goldfish plant_gunungkawi, Shutterstock
Image By: gunungkawi, Shutterstock

2. Not Researching the Needs of Goldfish

If you had fish as a kid, you’ve probably had the experience of standing in the aisle of the pet store, picking out cute tank décor, grabbing fish food and a heater, and heading home to get your new goldfish settled in. What many people don’t realize is that goldfish have specific needs.

A common mistake that people make is keeping goldfish in heated tanks. Goldfish are cool-water fish, which means if their home is in a climate-controlled environment, like your living room with air conditioning and heating, they likely won’t require a heater. This isn’t always the case, of course, but it’s true for most homes. Keeping your goldfish in warm water doesn’t sound like a huge deal, and for you, it may not be. The problem is that you may not realize that something is having a negative impact on your goldfish until much too late.

Keeping goldfish in warm-water environments can decrease their life expectancy, sometimes by years or decades. Providing a proper tank temperature for your goldfish is a major factor in ensuring that they’re alive for a long time.

Also, some goldfish, fancies especially, don’t do well with décor that has sharp or jagged edges. These rough areas can snag and tear at delicate fins, opening pathways for infection and stress.

The substrate that you choose for your goldfish is something else to consider when starting out. Fine substrates like sand or large substrates like large pebbles or river rocks are often the best options for goldfish. Some people even prefer to have no substrate for their goldfish.

3. Choosing Inappropriate Tank Mates

When it comes to choosing fish, many people use the “go to the store, pick out fish” method. What ends up happening is that they choose a fish based on appearance and don’t consider the species’ specific needs. So, if you pick out a goldfish and a tropical freshwater fish, like an angelfish, one of the species is going to live in less-than-ideal water parameters because goldfish prefer cool water while angelfish prefer warm water.

Some people get betta fish and goldfish without realizing the stress and danger that this causes both fish. Another common mistake with goldfish is choosing small tank mates. There is a bit of crossover between the environmental preferences of goldfish and guppies, for example, but goldfish will eat almost anything that fits in their mouth. This includes guppy fry and even adult guppies.

Some people claim that goldfish can’t be kept in a tank with any other fish because of how messy goldfish are. Fortunately, this isn’t true. There are appropriate tank mates for goldfish, including large snails, such as mystery snails, and other cool-water fish, such as dojo loaches. However, care should still be taken when choosing tank mates for your goldfish. Choosing inappropriate ones will likely end in heartbreak for you and stress for the fish that survive.

Black moor goldfish and common goldfish in home freshwater aquarium
Image By: Vlad Siaber, Shutterstock

4. Overstocking the Tank

This is a tough one because we’ve been told for so long that there are “rules” related to the size of the tank that a goldfish should be kept in. While there aren’t fixed rules, there are size considerations.

Consider the following guidelines as a good starting point:

For Fancy Goldfish
  • A single goldfish: 25 gallons (it is not advised to house them alone)
  • Two goldfish: 40 gallons
  • Three goldfish: 55 gallons
  • Four and beyond: an additional 10 or 15 gallons per additional goldfish
For Common or Long-Bodied Goldfish
  • A single goldfish: 55 gallons (it is not advised to house them alone)
  • Two goldfish: 75 gallons
  • Three goldfish: 90 gallons
  • Four and beyond: an additional 30 to 40 gallons per fish (a small pond setup is best for a large group of common variants of goldfish)

Though housing goldfish alone isn’t advised, we’ve provided recommendations in the unfortunate event that you need to isolate an individual for a closer inspection, medication, or treatment for an ailment or disease.

5. Under Filtering the Tank

Goldfish are extremely high bioload producers! This leads to a mistake that many people make when choosing tank filtration, and it’s easy to understand why.

If you have a 55-gallon tank and see a filter that’s rated for a 50-gallon tank, you may think that that’s “close enough.” When it comes to low bioload producers, you’re probably right. When it comes to goldfish, though, you’re definitely not right.

For goldfish, it is best to spring for a filter than handle at least twice the volume of the aquarium as a bare minimum. It is best to opt for a filter that can handle much larger volumes of water (such as five times the water volume).

With goldfish, it’s also advisable to have a powerful filter, such as a canister filter, in conjunction with a filter that provides more space for beneficial bacteria, such as a sponge filter. It’s hard to over-filtrate your tank, but you can easily under-filtrate it! Proper filtration removes visible and microscopic waste products, colonizes beneficial bacteria, and aerates the tank water.

Understanding the intricacies of water filtration can be tricky, so if you're a new or even experienced goldfish owner who wants a bit more detailed information on it, we recommend that you check out Amazon for the best-selling book, The Truth About Goldfish. 

The Truth About Goldfish New Edition

It covers all you need to know about creating the most ideal tank setup, goldfish care, and more!

green beautiful planted tropical freshwater aquarium with fishes
Image By: Sergii Figurnyi, Shutterstock

6. Making Poor Dietary Decisions

Just like all animals, goldfish require a balanced, nutritionally sound diet. The best base for your goldfish’s diet is commercial goldfish food because this is designed to meet their micronutrient needs. What this food doesn’t do is provide variety or balance. It provides the bare minimum of nutrition but not necessarily satiety. In the wild, goldfish and their cousins, Prussian carp, graze all day on aquatic plants and small animals that they come across, like freshwater shrimp. As you can imagine, fish food pellets twice a day may not satiate your goldfish the same way that grazing would.

Ideally, your goldfish’s food base should be pellets. Flakes are decent alternatives but tend to contain more fillers and fewer nutrients. Other food options to routinely include in your goldfish’s diet are gel foods, freeze-dried foods, frozen foods, and live foods. Ideally, your goldfish should always have access to plants to graze on. Goldfish that are allowed to consistently graze are less likely to develop swim bladder problems.

7. Doing Improper Tank Maintenance

Once your tank is cycled and your goldfish are settled in, you may think that it’s okay to do water changes every couple of months or so. However, nitrates will build up in your tank, and normal filter media will not remove them. A normal cycled tank will have some nitrates, with up to 20 ppm generally being considered safe.

If you aren’t performing water changes and don’t have a hundred plants in your tank, your nitrates likely aren’t going anywhere. This means they will continue to build up, to the detriment of your tank inhabitants. Routine water changes will help remove these excess nitrates.

The main issue with nitrates left unchecked is that, in high enough amounts, they too are toxic for fish and can lead to neurological issues. The only reason nitrates are not considered as deadly as ammonia or nitrite is because they have to be in very high amounts before they pose a threat to fish.

Another issue with excess nitrates in your tank is algae. This plant absorbs nitrates from the water for growth. In a well-balanced tank, your plants absorb most of the nitrates, and water changes take care of the rest. If you aren’t removing the excess nitrates, though, algae can get a foothold in your tank by consuming the nitrates that your plants aren’t using.

Algae isn’t just unsightly. It can grow to the point that it begins to outcompete other plants by consuming all the nutrients.

goldfish dirty tank_JenJ_Payless_Shutterstock
Image Credit: JenJ_Payless, Shutterstock

8. Knowing Treatment vs. Prevention

The number-one cause of illness in goldfish is poor water quality! Many people make the mistake of seeing their goldfish exhibiting signs of illness, so they dose them with medication. However, treating illness isn’t going to do any good if your water parameters are off and your water quality is poor. In fact, you’re likely just adding a stressor to an already stressful environment. Some sickly goldfish may not survive treatment with medication, so exposing them to this added stress during a time of illness that can be treated with a simple water change or water treatment can do more harm than good.

It’s also extremely important to keep in mind that drug-resistant bacteria exist. If you start dosing your goldfish with antibiotics that they don’t need or if you don’t complete a course of treatment after starting it, you’re increasing the risk for antibiotic resistance. Antibiotic-resistant infections are extremely difficult to treat, and even if your fish dies, you still may struggle to get the infectious organism out of your tank. Your best treatment of illness in your goldfish isn’t treatment at all; it’s prevention.

Properly caring for your tank, performing routine water changes, treating the water, and monitoring your parameters will do wonders for your goldfish.

9. Changing the Filter Media

If you read the instructions that come with your filter, you’ll likely see that the manufacturer recommends replacing filter cartridges every few weeks. Inexperienced goldfish keepers will usually stick to this, unintentionally crashing the tank’s cycle every time. Remember, beneficial bacteria live in the tank’s filter and filter media. This means every time you replace the filter cartridge, you’re removing a large portion of your beneficial bacteria.

In truth, your filter media should rarely be replaced. The advice given by manufacturers is to change the mechanical or chemical filtration portion of your filter. These include the initial cartridges where gunk is caught, as well as the carbon filtration. The “bio balls” or the biological filtration shouldn’t be changed, as this crashes the cycle. Beneficial bacteria are most concentrated in areas, such as sponges and any object with high surface area, oxygenated water, and decent water flow.

When you do water changes, it’s good practice to rinse it in the dirty tank water to remove the “gunk” without killing the beneficial bacteria. If you rinse your filter media in your kitchen sink under hot water, you’re killing your beneficial bacteria.

Most experienced goldfish keepers recommend that you replace the filter cartridges with long-lasting filter sponges and ceramic rings or beads that you can gently rinse from time to time without replacing them. This will help you get the most bang for your buck and ensure that you aren’t crashing your cycle every few weeks. It is also best to never rinse or change your entire filter’s contents at once, as this may lead to a crash in the nitrogen cycle.

aquarium cycle_hedgehog94_Shutterstock
Image Credit: hedgehog94, Shutterstock



It’s extremely easy to make mistakes as a goldfish keeper. Proper husbandry involves a great deal of knowledge and practice, which can take time to acquire. In this article, we’ve covered some of the more common pitfalls that newcomers to the fishkeeping hobby might encounter. Hopefully, the information provided here has better prepared you for your adventure in the world of fish-keeping.

Featured Image Credit: luckypic, Shutterstock

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