Understanding Water Quality and Recommendations for Stocking Your Goldfish Aquarium or Pond


Many fish keepers in general look for this magic number of fish per gallon rule to guide them on stocking their aquariums and ponds.  Over the years there has been many different suggestions. One of the oldest stocking guides still in widespread use dates to the 1960’s (before most tanks even had filters), 1 inch of adult fish length per 12 square inches of surface area.  Then came the more modern approach that many pet stores adopted in the 90’s 1 inch of adult fish length per US gallon of water.  In the early 2000’s many goldfish enthusiasts decided the best practice was 20 gal for the first Short Bodied goldfish and 10 gal for each additional fish.  One fish / 40 gal for the first long bodied goldfish and 20 gal for each additional fish.  Then in 2013-14 recommendations for goldfish became more extreme 20 gal for the first Short Bodied goldfish and 20 gal for each additional fish. Then one fish / 40 gal for the first long bodied goldfish and 40 gal for each additional fish.  These guidelines are a great way to help beginners in the hobby who are unfamiliar with the science behind water quality.  But the internet/forums/Facebook groups started taking these guidelines as law and the term “Goldfish Police” was born.  Over the years being involved with such groups and forums it’s been hard to watch people being chastised, chased off or just not sharing their pictures or videos of fish because they do not abide by these strict guidelines. 

In 2016 The Goldfish Council came into existence.  One duty the board took was creating a safe space for people to share their hobby without the fear of be ridiculed or judged on their set up or stocking.  One of the spaces created in Facebook Groups was The Goldfish Council – Goldfish Chat.  Here members try to help new and old members alike.  Sharing knowledge and just fun photos so we can all enjoy the hobby.  Personally, I have tried educating people in the group about water quality and how that plays a big role on how you stock your tank.  In this article I will cover some of the water quality basics, dive deeper into understanding the Nitrogen Cycle, and still provide some basic guidelines for beginners to follow if they are not experienced with maintaining an aquarium or pond.

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The first in-person Board meeting of The Goldfish Council in 2016

Understanding water quality can be quite the task, considering how many factors go into the results of your water quality.  Some of the Key factors I have listed below and will go into further during the article.  There are others factors not mentioned here that still play a role.  All these factors play a role in maintaining your water quality and work together or against your tank/pond like balancing act. 

  • Water source
  • Water Chemistry
  • The Aquatic Nitrogen Cycle
  • Filtration
  • Maintenance Routine
  • Feeding Regimen
  • Green Life Forms
  • Stocking Level
  • Volume of Water

Your water source can make thing very or easy or very difficult while maintaining a tank/pond.  You want to make sure you factor in the original chemistry of the water and source your water is coming from.  If your water is sourced from a municipal source generally there are added chemicals such a chlorine and chloramines added during treatment.   Always make sure you are using a good water conditioner that will bind these additives before adding fresh water to you tank/pond.   If your water source is from ground water on your property, generally collected from a well you will not have to worry about the additives.  Even if your water is derived from a well water you still might need to add conditioner based on the other chemicals that can be found do to agricultural and other fertilizers  leeching through the soil and effecting ground water.

I always recommend testing your water out of the tap and in a bucket or bowl after it has sat for 24 hours every day for a 5-7 days.  Gathering and evaluating your results can give you clear picture of what you are starting with before you ever add it to an aquarium/pond.  PH, Ammonia, Nitrite, Nitrates, GH and KH are the basics that can help determine what you are working with.  Once your tank is established you want to periodically test your water parameters, or if something seems off with the health of your fish a good place to start is a simple water test.  Below I have listed the basics in water chemistry.

  • The PH of water measures the acidity (hydrogen ion concentration) of your water. The pH scale ranges from 0 to 14 with 0 being the absolute most acidic water can be and 14 being the absolute most basic your water can be. pH balance, or a neutral pH value, is 7. Goldfish seem thrive in many different PH levels, but I have found the easiest range is 7.0 to 8.9. Maintaining a consistent pH level that is suitable to your fish’s needs is the first step towards consistently high-water quality.  The lower the PH is it can be harder on the fish during routine water changes.  Also, lower PH / more acidic water it can cause issues with breaking down hard matter like any rocks, decor, stock tank coatings.  Hobbyist must always remember that the nitrogen cycle consumes alkalinity and lowers the PH.
  • Ammonia is extremely toxic to fish and can quickly kill them. Unfortunately, it is naturally produced in all aquariums as waste and uneaten fish food breaks down. Ammonia can also be found in your direct water source. At no time do you want a reading of over 0ppm (part per million) in your tank.  There are many products on the market to bind ammonia and other harmful chemicals to assist you when doing water changes or topping off your tank/pond with fresh water. 
  • Nitrites are the result of ammonia being broken down by bacteria (Nitrosomonas sp) in your tank – they are the secondary product of the Nitrogen Cycle (which we will discuss further into this article). Nitrite is more toxic to your fish than ammonia. In new tanks, these levels will rise very shortly after ammonia levels level off and begin to drop. When you test for nitrates in your cycled tank, you ideally want a result of 0ppm.
  • Nitrates are the result of Nitrites being broken down even further by the beneficial bacteria (Nitrobacter sp.) in your filtration system – the third and final product of the Nitrogen Cycle. Nitrates are not particularly harmful to fish and their presence signals the complete cycling of your tank. In very high levels, however, they too can become a stress on fish. Nitrate levels should be low, between 20 and 40 ppm to remain in a safe range. The most common way decreases the level of nitrates in your tank, is to perform consistent water changes.
  • GH or General hardness is a measurement of the total dissolved minerals (calcium and magnesium) in your water. When your water has low amounts of dissolved minerals, it is considered soft. When it has higher amounts, it is considered hard. Soft water can cause pH crashes which can stress fish in your aquarium. The result of a water hardness test is measured in degrees (DH) and an acceptable level for goldfish is generally between 100-300ppm.
  • KH or carbon hardness is an alkalinity test used to determines your water’s ability to maintain PH.  This test is an indicator of how stable your water quality is. Low alkalinity levels mean that your pH will fluctuate more easily which can put stress on your fish. A low level also will stunt the growth of any live plants in the tank. For most community aquariums, a good alkalinity level should probably read between 7 and 12 degrees (dKH).

Once you have a general understanding of your water’s chemistry, you will need to know how the nitrogen cycle works.  The nitrogen cycle is what takes place in the biological filtration. It also takes place on any hard surface including substrate, sides of your aquarium all of this takes time to develop and grow. Colonies of nitrifying bacteria grow and help eliminate harmful naturally occurring toxins that form in your water column from fish waste and other organics. Goldfish produce large amounts of waste in comparison to other aquarium fish.

  • Ammonia generally enters your tank or pond through disguised as protein in your fish’s food.
  • Waste or organics break down creating Ammonia (ammonia is toxic to Goldfish).
  • Nitrosomonas Bacteria then break down ammonia into Nitrite (like ammonia, Nitrite is very toxic to goldfish).
  • Another type of beneficial bacteria called Nitrobacter Bacteria it breaks down Nitrite into Nitrate.
  • Nitrate is not as harmful and can be tolerated by fish up to 40ppm.  Ideally, we want our nitrates to be 20ppm and under. Nitrate then then be removed via water changes. Plants also help keep nitrates low, many established tanks with plants seldom have nitrates over 5 ppm.
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Image credit: www.animates.co.nz/articles/the-nitrogen-cycle

Filtration plays a big role in maintaining your water quality. The three main types are mechanical, chemical, and biological filtration.  Having adequate filtration or over adequate filtration can cut down on the amount of weekly maintenance you will have to perform to keep you water quality up. 

  • Mechanical filtration is designed to catch particles (solid waste) suspended in the water in your aquarium. Water is forced through filter media which needs to be cleaned on a regular basis. Most clean the filter media in tank water or replace it every 2-4 weeks.
  • Chemical filtration occurs when toxic chemicals pass through a resin or media. Some chemical filtration products target specific excessive nutrients or chemicals from the aquarium. Chemical Filtration is used less and less by many hobbyists, especially Activated Carbon. Some believe if Activated Carbon if not changed religiously the buildup of toxins can be leached back into the water column causing more harm than good.
  • Biological filtration is a space to house beneficial bacteria and as water passed through the media which helps nitrogen cycle work more effectively. Biological filtration is the most important part of filtration for your goldfish’s health. Making sure you provide ample space for the bacteria to grow on.

Common types of filtration have been listed below.  Things to keep in Mind when selecting your filtration is to when the box reads it is recommended for 40g tank it will only be good for 20g when it comes to goldfish due to the increased waste in comparison to other aquarium fish. It is recommended finding one that cycles the water 8-10 times the tan/pond volume in one hour. For example, 40g /150 l would need a filter with a flow rate of 320 g/1200 l per hour. This ensures the beneficial bacteria in your filter media and efficiently do their job.

  • Canister Filter- These are larger units that are generally placed under the aquarium, with an input and output going into the aquarium. It provides lots of room for your mechanical and biological filtration, some even come with UV sterilizers for added protection against harmful protozoans and bacteria. Usually additional aeration is added with air stones or wave makers when using canister filters because most of the time do not create enough surface agitation to create more surface area for oxygen to take place.
  • Sponge Filter- Underrated and very cost effective, one of the best for biological filtration. These are either driven by air pumps or powerheads which also creates great surface agitation. The downside to these is they do not do as great of a job with mechanical filtration.
  • Hang-on Back Filter- The most common style filters for beginner and experienced goldfish hobbyists alike. They commonly used because they provide excellent mechanical, chemical and biological filtration. These filters hang over the back of your aquarium and suck water up through their siphon tube. Then the water passing through a filter pad or floss for mechanical filtration to take place. Next chemical filtration occurs from water flowing through carbon which removes toxins or chemicals from the water. Then lastly biological filtration happens inside the filter cartridge. Large colonies of beneficial bacteria form within the filter cartridge, biological filter pad and or bio-wheels.
  • Wet/Dry and Trickle Filters- These filters are not as common as the previous filters mentioned above but some goldfish hobbyists use them (common in saltwater tanks). They do excel in the biological part of filtration due to part of the filter media is exposed to aquarium water and a significant amount to air. Being exposed to both air and water creates a large colony of beneficial bacteria to grow and process waste products. Setting up this system requires a lot of work including getting plumbed directly to your aquarium with a sump or reservoir to hold auxiliary water and water to run the pump. Mechanical filtration is limited as water must pass through large-pored sponges, so the water flow is not restricted like other forms of mechanical filtration.
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Examples of filter types

Maintenance is a requirement for keeping goldfish.  Maintenance includes but not limited to water changes, cleaning the filtration, vacuuming, scrubbing the walls of the glass & water additives.   Goldfish hobbyists perform water changes for several different reasons, the first one is to remove waste. When we test our water (with our liquid test kit), we test for Ammonia, Nitrite and Nitrates. If ammonia or nitrite are present, we must perform a water change. A cycled tank would not have a reading for either of these as they would have been converted by the nitrifying bacteria. These toxins, even in small amounts can cause stress, induce illness and disease and even cause fatality. You would have a reading for ammonia and nitrite if your tank is either not cycled or is going through a cycle bump. Water changes and daily monitoring are vital during this time. Once your tank/pond is fully cycled water changes are one way of removing excess nitrates.  Nitrates are not as harmful as ammonia or nitrites but still but be kept at a respectful level to keep your goldfish healthy and happy.  Cleaning the filter and media is necessary some keepers clean these out bi-weekly or monthly just depends on the size and type of your filter.  Cleaning the media in tank water is the best practice to keep harmful water additives from killing of your beneficial bacteria. By cleaning your filtration, you are removing build up of waste and solid organics that build up.   If a buildup of these organics goes unchecked, they can wreak havoc on your cycle and become a source for harmful bacteria.  For the same reason you will also want to vacuum your substrate or bar bare bottom tank removing buildup of solid organic matter.  Scrubbing the walls of your display tanks is a personal preference.  Some goldfish keepers like all the walls on their display tanks to be spotless removing all forms or algae growth.  While others like myself,  only scrub the front panel of the tank to be able to observe the fish allowing algae to grow on all the other sides.  I will go into more depth on algae growth when we discuss green life forms.  Water additives other than conditioner are not always necessary.  Depending on your water’s chemistry sometimes it can be necessary to add compounds that raise your Ph, help your waters buffering compacity, or boost your beneficial bacteria colony.  I always recommend discussing this with an experienced hobbyist before you start attempting using certain additives as a regular part of your maintenance routine.

How often you preform the above-mentioned maintenance depends on factors such as stocking levels, volume of water, your waters original source, and other water quality factors discussed in this article.  To develop a routine that works for you will all depend on your overall water quality.  So regular water testing is needed until you find what works for you, your lifestyle, and the health of your Goldfish.  

Feeding regimen is an important factor in keeping your goldfish healthy.  Making sure your fish have the essential nutrients, minerals, & vitamins is key.  I will not be discussing in depth the components necessary in their diet but will more so cover how your feeding regime effects water quality.   The amount of food and types od food you provide your fish directly effects the water quality.  Over feeding can create water quality issues by creating excess waste in your tank/pond.  Uneaten food turns into harmful organics in the water creating additional ammonia for your cycle to break down eventually raising your nitrate levels.  Also, the more you feed the more waste a goldfish produces which again raises your nitrate levels.  Finding a balance that fits with your maintenance schedule and keeping your fish well feed is a balance you will have to explore. Remember the higher the protein content of the food, the more ammonia ends up in your tank. Often you will hear experienced goldfish hobbyist referring to the term grooming.  Nutrition and the volume of food are a big tool used when grooming your fish.  These factors directly impact the goldfish’s overall growth and development, so you will want to keep that in mind as well when developing your feeding regimen.

The term green life forms refer to plants & algae growing submerged or floating in your aquarium/pond.  Green life forms can assist with the removal of nitrates (the end result of the nitrogen cycle).  These green life forms are one of the only other ways to eliminate the buildup of nitrates in your system besides manual removal through water changes (Denitrification is not discussed in this article, nitrate to nitrogen gas).   Algae build up on the walls of the tank/pond are a good source of green life form they require little no care and build up over times with only a little light and nitrates need to thrive.  Free floating algae is another form of algae that is found in the actual water column, when found at reasonable levels is referred to as green water.  Many experienced hobbyists keep their fish they are growing out in green water.   The free-floating algae helps eliminate the nitrates and while simultaneously providing nutrition and intensification of the fish coloration. Goldfish consume just by pushing the algae into digestive track through their gill rakers.  Submersible plants are a little tricky with goldfish. Since goldfish are natural omnivores, they love to eat plants.  Finding plants that your goldfish will not eat is part that becomes tricky, you will often find different goldfish will have different taste for different varieties of plants.   Some submersible plants I have personally had luck with are Anubias, Java Fern , Cryptocoryne, and some varieties on Valisineria.  I recommend only buying a few plants to start with until you figure out what will work with your fish, your lighting, and your substrate or lack of substrate.  Floating and emergent plants is another option you can work with in providing green life forms for your tank/pond.  Floating plants are just that, the plant floats at the water surface and root remain in the water free floating around the water source.  Examples of these that I have seen work well with goldfish are Water Lettuce, Frog Bite, Hornwort, Anacharis and Water Hyacinth.   Emergent plants are rooted in a form of substrate break the water surface and continue new growth above the water’s surface.  Examples of these are Water Lilies, varieties of Iris, Creeping Jenny, Bamboo, and Cattails.  Make sure you do your research; some types of floating and emergent plants are illegal in some states here in the US because of their invasive nature.    These varieties of plants are typically used more in ponds but can be adapted to use in the aquarium with the right lighting and open top aquariums.

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A planted tank with some varieties of goldfish safe plants

How your stock your aquarium or pond in relation to the total volume of water your system holds is one of the biggest factors in determining your overall all water quality.  The lower your stocking density the easier it is to keep your water quality in tip top shape.  The heavier the tank/pond is stocked the more difficult it is to maintain.  I have come up with some very basic recommendations to give new hobbyist and idea of what experienced hobbyists have found over the years keep fish healthy and happy. Keep in mind these are not rules by any means.  They are merely a suggestion to give you an idea how to stock or help you planning on setting up a new tank or pond.   I have outlined common practices by Beginners, Advanced keepers, & Goldfish Breeders. Another thing to keep in mind is some fish grow larger than others and may need more swimming room than the basic recommendations suggest.  

The  recommendations below are a safe rule of thumb to follow to insure the overall health, quality of life, and vitality of your Goldfish: Most goldfish keepers & breeders keep a simple tank minimal décor that is goldfish safe, bare bottom or sand/fine gravel for substrate, planted or unplanted, with double the filtration recommended for the volume of water the filter is rated for. These recommendations can be modified with more frequent water changes, Larger and more effective filtration, closer monitoring of the water quality and other factors mentioned in this article.  Keep in mind some advanced keepers & breeders often move fish around quite frequently, so most tanks are never their forever home.

 Beginner KeepersAdvanced KeepersBreeders
Fancy Goldfish20 g /75 l per fish and 15 g /55 l for each additional fancy goldfish added40 g /150 l per fish and 30 g /110 l for each additional common goldfish added20 g /75 l per fish and 10 g/ 35 l for each additional fancy goldfish added
Pond/Long-Bodied Goldfish40 g /150 l per fish and 30 g /110 l for each additional common goldfish added40 g /150 l per fish and 30 g /110 l for each additional common goldfish added20 g /75 l per fish and 10 g/ 35 l for each additional fancy goldfish added
Juvenile Goldfish 20 g /75 l per fish and 10 g/ 35 l for each additional fancy goldfish added20 g /75 l per fish and 10 g/ 35 l for each additional fancy goldfish added
Goldfish Fry  kept in smaller water volume based on size of spawn and variety of goldfish but get more frequent water changes
Basic recommendations

I hope this article has shed a light and opened your mind to some of the factors that contribute to the over all water quality we face as goldfish hobbyists.   If you are new to the hobby, you find over time even though it is a lot to take in it gets easier the longer you do it.  Also experienced hobbyists find new ways to achieve optimal water quality all the time playing with these factors and experimenting.  Think if water quality as balancing of the scales, finding that balance is what is key in maintaining great water quality.  I personally have had many successes and failures in the goldfish hobby.  Learning from each of these hit or misses has kept my mind open to new possibilities.  The key to enjoying the hobby is finding a balance between these factors and what works for you and your lifestyle.  Always remember water quality for our fish is the number one way to keep them vibrant and healthy.

One of my Butterfly Telescopes during feeding time

©The Goldfish Council 2020