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Goldfish 101

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My best friend in London got in touch with me - a friend of hers was having problems with her fish so she'd called me for help. Usual story - small tank, no filter, 1x10% water change and she'd had the fish a week. She's now getting a filter and heater, upgrading her tank and doing daily 80% water changes until further notice. I've prepped a care package and also typed up a Goldfish Basics Guide to get her started, you guys will all know a lot of it but I thought it might be helpful to have it all in one place. There's obviously plenty that could be added to it, this is meant for someone completely new to goldfish, and I know some people come straight to the forums without going to the main site and there's a lot of information to sift through, so I thought having it in one place could be helpful. This is aimed at someone who's upgrading from a 5 gallon to a 10 gallon (she doesn't have space for a 20 gallon at the moment but is moving in a few months) for two 5cm telescope goldfish and is based from a UK perspective on my experiences with goldfish.

The Tank

Tanks can be quite a big expense, particularly if you end up upgrading through several sizes up to whatever your final tank size ends up being. Based on adult sizes (about 5-6 inches in body for most pet store fish which are often stunted when bought and therefore don’t tend to grow as massive as some) you need 10-15 gallons (40-60 litres) per fish, and the more the better and it can work out cheaper in the long run to get the biggest tank you can fit/afford. This is due to large amount of waste comparative to their size that goldfish produce – you need that much water to dilute it down so that it becomes toxic much more slowly and won’t harm the fish. More on that later!

Tanks come in an assortment of different shapes and sizes, acrylic and glass. Acrylic is lighter but glass doesn’t scratch as easily. Plastic tanks tend to be more common in the smaller sizes, glass for larger and both are available second hand from Ebay or Gumtree. For your two fish you ideally want a 20 gallon (80 litre) tank, but while they’re small if you’re able to keep up with water changes and get a decent filter then they will be fine in a 40 litre tank for a while.

Tank Equipment and Water Chemistry

Filters are basically where your goldfish care ecosystem will live. The Nitrogen Cycle, or just Cycle, is something you will hear about in pet-stores and on the net and it basically boils down to poop processing.


The fish poop is broken down by bacteria into ammonia and this is then broken down by another bacteria into nitrites. Both are toxic even in low levels to fish, ammonia is the nastier of the two as it can cause burns on the body and gills if allowed to get too high and both should be kept below 0.25ppm (0.25g/l) at all times. A third bacteria turns the nitrites into nitrates which are less toxic to fish – levels should be kept below 20ppm (20g/l) for health reasons, but below 10ppm is better. The nitrates to nitrogen stage should not be seen in an aquarium as it is changed by an anaerobic bacteria (no oxygen present) and these are usually nasty to fish and take place in stagnant water. You can keep all these chemicals down by changing the water – if you remove 50% of the water you lower the level by half of each component. Nitrates can also be controlled by adding aquatic plants which will absorb some of them for growth.

These bacteria take time to grow and you will see this by a change in the levels of each chemical which can be tested with your test kit, to start with when the water is clean (and the tank new) all the readings should be 0ppm, assuming this is the case with your tap water – it’s important to check your tap water levels so you know what you’re putting into the tank. The first to show a result will be the ammonia, and as the nitrite levels appear and then the nitrates this will eventually disappear until eventually the readings for ammonia and nitrite are zero and there is a nitrates reading which will increase with time. While the ammonia and nitrite levels are changing it is important to check these daily and do water changes accordingly – don’t let them get above 0.25ppm or they can start to have negative effects on the fish. When the cycle is stable and you’re getting a steady nitrate reading the testing can be backed off to once a week, and the same with water changes – as long as the nitrate levels are low enough after this period of time. Stabilising the cycle can take anything from a few weeks to a few months.

The filters contain media for these bacteria to grow on, and you can get a range of different types of media – broadly classed as sponge and ceramics. It’s good to have a mix of both in your filters, the sponges will trap muck into the filter for processing and the ceramics have a very large surface area for bacterial growth. The ceramics also provide a permanent source of bacteria, as the sponges will need to be rinsed out of muck about once a month in tank water in a bucket. Tap water (with chlorine in) will kill off the bacteria. Sponges can be thrown away and replaced when they become very mucky, but you shouldn’t throw them all away at once as you will be throwing bacteria away at the same time and will see a cycle bump (ammonia and nitrites will appear again) as the remaining bacteria try to deal with the bioload.


Filter sponge in the UK can be really expensive for small quantities, a pain if you have a big filter. The best place I’ve found for it is sourcingmap.com, I use a mix of this one http://www.sourcingmap.com/layer-aquarium-fish-tank-biochemical-economic-filter-sponge-blue-p-68840.html and this one http://www.sourcingmap.com/aquarium-fish-tank-biochemical-sponge-yellow-filter-p-62305.html so that I get a mix of coarse and finer sponge filtration, and I just cut it to size with scissors. Media set up should be with the coarsest sponge closest to wherever the water is coming into the filter, with the finer sponge below it, and the ceramic media just before the outlet so that the muck is kept away from the high bacteria density in the sponges. The chemicals the bacteria consume are waterborne and invisible to the naked eye, they won’t actually break down the lumps of algae, poop and leftover food that will get into the filter.

There are different filter types – the most common being internals, hang-on-backs (HOBs) and canisters (a.k.a. external filters). Internals are most common for smaller tanks and are usually the cheapest, canisters are for larger tanks and are the most expensive, with HOBs falling somewhere in the middle. I keep a canister filter on my goldfish tank, and an internal on my Siamese fighter fish tank.


Internal and HOB filters should be sized to give 10x your tank volume in flowrate per hour (so a 40 litre tank will have a filter with 400 lph), canister filters can be a little smaller – 5-7x flowrate – as they have such a massive volume of filter media for the water to pass through, so it needs fewer passes to be cleaned by the bacteria.

Water changes are not only important for keeping the waste and toxin levels down, goldfish produce a hormone into the water, particularly when stressed, and if this level rises too much they will stop growing – on the outside at least, and this leads to stunting. It’s very common in bulk breeder and fish store tanks as there are so many fish in such a small volume of water. The only way to remove this hormone is through changing the water and this helps prevent the fish becoming stunted. Fish that are already stunted will still grow but at a slower rate and usually not as much as those who are not stunted when they come home to an aquarium.

It’s important to check the ammonia, nitrite and nitrate levels in the tap water, but it’s also important to know the pH as swings in pH (even if they don’t look that big) can damage the fish’s slimecoat and make them ill.

When changing the water it’s important to make sure the temperature and the pH of the tap water going in match what’s already in the tank or you’ll shock the fish. Goldfish like temps of around 24-26’C (just make sure it’s constant by using a heater) and pH of around 7-8 – again, constant is really important. Once the cycle’s established, if the tank is not overstocked 50% once a week is a usual amount of water to be changing, but until then daily water changes of however much is necessary (this can be as much as 80% twice a day) will be needed to keep the toxins down while your filter bacteria sort themselves out. Always use dechlorinator, this prevents the chlorine in the tap water burning the fish’s gills. One of the best dechlorinators around is SeaChem Prime, it can be bought most cheaply on Ebay and is a higher concentration than many you find in pet stores so one bottle lasts a long time. Prime also has the benefit of binding ammonia for 24 hours into a less toxic form what can still be consumed by the filter bacteria, so it’s particularly useful when cycling, but it won’t replace water changes.


A really useful gadget for water changes is the BiOrb cleaning siphon, which you can get on Amazon or in Pets at Home – it’s a hand-pump started siphon which is really useful for removing muck from the bottom of the tank and getting water out the tank into buckets without getting it everywhere.


Goldfish are also high oxygen users – you may see them gasping at the surface occasionally. Aim the filter outlet across the surface to cause waves if possible to help increase oxygen interaction, and also look into getting an airpump with an airstone or bubble wand. The airpumps I prefer are TetraTec’s, they’re very quiet compared to some and the APS50 should be big enough for a 40 litre tank. Ebay is a good source for one-way valves if there isn’t one with the pump and this will stop any water getting into the pump and breaking it.


Lighting is not essential for keeping goldfish, though it’s nice to be able to see them and it will be necessary for keeping real plants if you want to. You only need to be concerned about the type of light (wattage and colour spectrum) if keeping live plants and most of the fussier plants will be eaten by goldfish anyway, the goldfish-friendly plants (java fern and anubias) will thrive in most light conditions.

Testing the Water

Add 5ml (to the fill line) of water (usually tank but occasionally check the tap water too) to each of the 4 test tubes in the kit.

Ammonia – add 8 drops of bottle A, shake the tube, and then shake bottle B, add 8 drops of bottle B to the tube, shake again and leave for 2-3 minutes.

Nitrites – add 5 drops of solution

Nitrates – add 10 drops of bottle A, shake the test tube, shake bottle B for 30 seconds, add 10 drops to test tube, shake the test tube for 1 minute, leave for 2-3 minutes

pH – add 3 drops of normal pH indicator OR 5 drops of high pH indicator

Compare all the test tubes with the colour charts to see what your levels are and react accordingly.

New test kits can be bought on Amazon, the best is the API Freshwater Master Test Kit as liquid drop tests are more accurate than the paper tests you can find some places.


Tank Decorations

Goldfish are really messy – therefore gravel should either be kept to a very thin layer (just enough to cover the bottom) so there’s no poop build up underneath, or left out entirely and the bottom of the tank left bare or scattered with glass stones or river pebbles. Deep gravel layers do not get access to oxygenated water, so the bacteria that break down the muck trapped in the gravel (which is much harder to clean and remove) are anaerobic and produce hydrogen sulphide, which leaches into the water and is very toxic. This can also build up in stagnant water areas so it’s best to avoid hollow decorations too. There’re plenty of decorations which can be used, if you want artificial plants silk is better than plastic as it’s got less sharp edges which can damage fins, tails and eyes, and any resin decorations (shipwrecks, Spongebob, etc.) should also be kept free of pointy bits, especially with telescopes. Sourcingmap.com is another good place for cheap aquarium fake plants and decorations. For a more natural look, real plants such as Java Ferns and Anubias are the best for goldfish, as anything delicate gets eaten. If you want to give your goldfish a healthy treat, pondweed is a very fast grower, and as such consumes nitrates very quickly (a plus!) and is welcomed as a floating salad bar by most goldfish. Terracotta plant pots can be added to your tank as long as they are not glazed – many glazes contain metals such as lead which will leach into the water. Java ferns and Anubias do not like to have their roots buried so are also good for bare-bottom tanks – they prefer to be tied to rocks or driftwood.


I prefer my tank with the natural look personally, it’s pretty busy with driftwood and plants but there are other people who keep their tanks really empty with just a few key focus points, which is particularly good for smaller tanks as the decorations will take away from the water volume available for waste dilution and swimming space.



Goldfish will eat anything that fits in their mouths (which is why they shouldn’t really be kept with anything else – smaller fish will be eaten!) and they’re also omnivores. The simplest food is to get a good quality sinking pellet like Saki-Hikari, which is high in protein and low in filler ingredients, and then supplement this with peas, cucumber, lettuce, spinach, garlic, fruit (less than the veg though), anything really just lightly blanched in hot water to soften it and then either chopped into bite size pieces or you can get vegeclips which suction cup onto the side of the tank for leafy veg (Ebay is easiest). Stay away from broccoli and sweetcorn though as these are gassy vegetables for fish and this can lead to buoyancy problems. Small fish will only need about 5 of these pellets each once a day. A good guideline for feeding goldfish is never more than the size of their eye (and for telescopes this is just the circular area of the seeing bit not the whole orb!), and remember that they have very twisty stomachs and get constipated very easy – so small portions more often are much better for them.

Feeding too much at once can cause problems for the fish, but it also results in extra waste which will impact your water quality, particularly when cycling the tank feed only sparsely to reduce waste. Goldfish can go as long as a week without food (so you don’t need to worry about feeding them when you’re away for a weekend) as long as they’re healthy, and it’s actually good for them to fast one day a week.


Goldfish Illnesses

There are many things goldfish can come down with, but this is just the most common ailments, particularly in new fish (hopefully you won’t have to deal with any of them).


Sadly when keeping goldfish it’s very important to pay attention to their poop – it’s a very good indicator of stress, diet problems and illness. Healthy poop should be short, fat, straight, food coloured and free from bubbles, and shouldn’t trail after the fish. My usual rule is if you’ve not seen your goldies pooping, and there’s none floating around the tank, they’re usually healthy as healthy poop should be quickly released and quickly sucked up by the filter. If the poop is trailing after the fish and food coloured, with intermittent stringy bits this is a sign of constipation – reduce the amount of feeding generally, fast the fish for 2 days and then feed peas only for 2 days. You should start to see healthy green poops appearing and then can return to normal food. If this happens often reduce the amount of food at each feeding. If the constipation appears really bad and peas are not helping, a single grain of Epsom salt in a pea fed to the fish will act as a laxative but this should not be used more than once per session, and Epsom can be harsh if used for too long.


Ich (or ick) is an external parasite that shows up on the fish as little white spots like grains of salt or sugar. It can be treated by raising the temperature of the tank water to 80°F slowly over several hours 1 degree at a time to prevent heat shock and using aquarium salt. The increased heat speeds up the life cycle of the ich, as it can only be attacked by the aquarium salt when not attached to the fish – when it has fallen off as an adult, or when it has hatched from the egg and not yet attached to the host. Aquarium salt must be carefully dosed over a period of time at 1 teaspoon per gallon of water (approximately 4 litres to 1 gallon). A 40 litre tank will require 10 teaspoons of aquarium salt to raise the salt level to 0.1% salt. When doing water changes, add back in the amount of salt related to the water you’ve removed – if you’ve removed 50% of the water, you’ve removed 50% of the salt and therefore need to add this back in when you’re done. Ich is cured by increasing the salt level 0.1% at a time to 0.3% over a few days – 2 days at 0.1% (10 teaspoons in the tank), 2 days at 0.2% (20 teaspoons in the tank) and then at 0.3% (30 teaspoons in a 40 litre tank) for as long as it takes for the last grain to be gone from the fish – then reduce the level to 0.2% for a further week to ensure all eggs and hatchlings are killed before reducing the level again to 0.1% for another week and then down to zero.


If the grains are appearing only on the gillplates, this can be a sign of a male fish reaching maturity (and size is not an indicator of age in store fish due to stunting). This is NOT ich but breeding stars and does not need to be treated with salt.


It can be good practice as part of the quarantine procedure for new fish to treat them with aquarium salt at 0.1% for a week or two to check for an ich breakout – as it can lie dormant and you want to deal with it before introducing it to your current system. With a new tank, it is more important if there are no symptoms of ich to concentrate on setting the cycle up first as the salt can have a negative effect on the filter bacteria.


Flukes are a gill parasite that can be noticed in fish by behaviour known as flashing – very quick flicking along the bottom of the tank as though they have an itch, sometimes as though rubbing against the floor. Flukes are easily treated with an ingredient called Prazi in a medication available in the UK as Fluke-Solve. It’s very mild on the filter bacteria and the fish and very good for treating flukes. I’ve only seen it available from www.wormerplus.co.uk where they do next day delivery. This is another medication that’s good for quarantine procedures but again, with a new setup concentrate on cycling unless you see symptoms and you can treat later as a precaution if you want to.


There are many other goldfish diseases, infections, nasty things that can happen but hopefully you won’t have to deal with them. Get to know your fish and their normal appearance and behaviours and you’ll know when something’s wrong. Let me know if you’re not sure on anything :)

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oooh! so informative!! thank you for all this valuable information hun.. i will be sure to refer people to this page as required :heart

good work :)

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Well done. What a great article. Is that a picture of your tank??? It's really fantastic. That's kind of the look I'm going for, it gives me some inspiration. :)

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Thanks guys :)

That is my tank, about 7 months ago, there's a lot more plant growth now (and I'm expecting another foliage boom when I get around to changing the bulbs), it's not a huge amount taller in most places, but it's much denser, you can't really see through it :)

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Great article that will no doubt be a life saver to a person new to keeping goldfish :) If i was to be really really picky i would point out a couple of errors that you have made and may want to change. Please don't get mad, i just can't help myself :(

Firstly - I would change the recommended tank size (I know that you originally did this for a friend)but if new fishkeepers are going to refer to this i would state that based on keeping 2 small goldfish a 30 gallon tank should be the minimum size of tank to buy but even then they may have to upgrade as the fish grow. People should be aware that when buying a tank you should always buy the size of tank that will suit the fish when they become fully grown.

Secondly - When you describe the Nitrogen Cycle you state that it's all about poop processing. Poop and uneaten/decaying food do contribute to the Nitrogen Cycle although they play a very small part. The majority of ammonia produced that contribute to the cycle is actually from the fishes gills (respiration). A fish releases all the toxins through its gills as opposed to peeing it out like we do.

Finally - When speaking about good goldfish food you mention Hikari. I have used this product for years but have now stopped as i believe that it is now considered to be not so good for goldfish. I think i read a post on here stating that it was full of MSG and i think alot of people are actually switching to different brands. Frustrating really as Pro-Gold and Omega One are not available to buy in the UK

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