04 Aquarium
The goldfish aquarium
Build a better environment. Goldfish are large, messy, cold-water fish with particular needs, and an aquarium designed around those needs is the difference between surviving and thriving.
- Filter rated well above the tank’s volume, outflow softened for fancies.
- Air stone or surface agitation so oxygen can enter and carbon dioxide leave.
- Sand or bare bottom. Small gravel can be swallowed and lodge in the throat.
- Smooth decor with no sharp edges or gaps a fish could wedge into.
- Hardy plants such as anubias and java fern, attached to rock or wood.
- Thermometer where you can read it daily; avoiding large, fast swings matters more than the exact number.
- Lid and light on a timer for six to eight hours a day.
The ideal goldfish aquarium
A walk through a goldfish aquarium built the way we would build it, wide rather than tall, filtered generously, planted with what goldfish leave alone, and easy to keep clean. Read it first · 12 minContents
29 guides, written to be read in full.
Choosing and setting up 9
- Choosing a goldfish tank The tank is the one purchase you cannot easily upgrade later. Here is how to pick a shape, a material and a size that will still suit your goldfish once it is fully grown. 13 min
- Substrate for goldfish tanks: sand, gravel or bare bottom? Goldfish spend their days mouthing whatever is on the floor of the tank, which rules some substrates out and makes others easier to keep clean than you might expect. 11 min
- Plants for goldfish tanks: what survives Goldfish treat most plants as salad. A handful of species are tough enough to be left alone, a few more are worth growing as food, and one houseplant does the best work of all. 13 min
- Aquarium lighting for goldfish Goldfish do not need bright light. They need a predictable day and night, and a modest LED on a timer covers viewing, easy plants and the fish's own rhythm. 12 min
- Safe aquarium decorations for goldfish Goldfish are clumsy, curious and surprisingly fragile at the edges. The right decor is smooth, stable, inert, and leaves most of the tank as open water. 12 min
- Setting up a hospital tank Many pages on this site tell you to treat a fish in a separate tank. This is the tank: bare, sponge-filtered, heated, lidded, and run on water changes rather than on biology, because some of what you put in it will damage the filter. 14 min
- Tank placement, stands and floor loading A 200 litre goldfish tank loaded and running weighs around 270 kilograms, concentrated on the footprint of a small desk. Everything else on this site pushes you toward a tank that size; this is the page about where it can actually go. 15 min
- Aquascaping for goldfish Aquascaping is built around fish that stay small, leave plants alone and never dig. Goldfish do the opposite of all three. The constraints are unusual enough to be interesting, and a goldfish tank designed around them can look very good. 11 min
- Buying second-hand aquarium equipment Second-hand is the single biggest lever on whether a goldfish gets enough water, because the tank it needs is expensive new and cheap used. The filter is both the best buy and the riskiest, and the decision it forces is the whole of this page. 11 min
Filtration, air and temperature 7
- Goldfish filtration: types, sizing and flow Goldfish need a filter with more capacity than their tank size suggests, and an outflow gentler than most filters deliver out of the box. Here is how to get both. 13 min
- Filter media: what goes in, in what order, and what never to throw away A filter is a box with a pump. What you put in it is the actual filter, and a common mistake is throwing the important part away on a monthly schedule because the packaging said to. 13 min
- Aeration and oxygen in a goldfish aquarium Oxygen enters an aquarium mainly at the surface, and goldfish are big, heavily fed fish that use a lot of it. Here is how to keep the supply ahead of the demand. 12 min
- Heaters and cooling a goldfish tank Most goldfish tanks do not need heating for warmth. Where a heater earns its place is stopping a cold room dragging the tank around — and in summer the problem reverses, which is the half nobody plans for. 14 min
- Power cuts and aquarium emergencies In a power cut the thing people worry about is the cold, and the thing that can do the damage is oxygen — for the fish first and for the filter bacteria straight after. Goldfish tolerate low oxygen better than many aquarium fish, but at room temperature that means hours, which is time to do the right things. 12 min
- Automating a goldfish tank: what is worth it, and what hides a problem The best return from automation on a goldfish tank is not labour saved but failure noticed. A timer and a temperature alarm are worth more than everything else combined, and an automatic feeder is the one piece most likely to cause the problem it was bought to prevent. 11 min
- Sumps and large systems A sump is not better filtration. It is more water and somewhere to put the equipment — both of which are worth a great deal to a goldfish. The price is that it converts a sealed system into a plumbed one, and the sum that decides whether that is safe is one almost nobody does. 12 min
Water quality 4
- How to do a water change, step by step The water change is one of the most useful things you do for a goldfish, and it takes twenty minutes once the routine is in place. Here is the routine, step by step. 12 min
- Testing goldfish water: ammonia, nitrite, nitrate and what the numbers mean Most goldfish problems start in the water, and none of them are visible. A test kit turns guesswork into numbers; this guide explains which numbers matter and what to do about them. 13 min
- pH, KH and GH in a goldfish tank Three readings that most keepers treat as independent are really one system. KH governs whether pH holds, pH governs how poisonous the ammonia in your tank is, and GH is mostly along for the ride. Here is how they work together. 15 min
- Tap water, dechlorinator and source water Your tank's water is your tap water plus whatever you did to it. Almost nobody looks at what they are starting with, and several of the most stubborn problems in fishkeeping begin at the tap rather than in the tank. 14 min
Keeping it clean and clear 4
- Cleaning a goldfish tank without harming it A goldfish tank is cleaned little and often, and half the skill is knowing what not to touch. The bacteria in the filter do most of the work; your job is to help them. 12 min
- Algae in a goldfish tank, and green water Every lit tank with fish in it grows algae. The useful question is not how to eliminate it but which kind you have, because each one is telling you something different about the light and the nutrients. 12 min
- Cloudy water in a goldfish tank Cloudy water is four or five different problems that happen to look alike. The colour and the timing tell you which, and a common one is a bacterial bloom that usually clears on its own if you leave the filter alone. 11 min
- Snails, hitchhikers and biofilm Three things turn up in a new aquarium that nobody warns you about, and all three get mistaken for problems. Most hitchhiker population booms are not an infestation; they are often a nutrient reading you did not ask for. 11 min
Outdoors 4
- Keeping goldfish in an outdoor pond A pond is where a common goldfish or comet finally gets the space its body was built for. It is also a different kind of keeping, with seasons, predators and a rhythm of its own. 18 min
- Building a goldfish pond The pond guide sets out what a goldfish pond needs. This is how to make one — starting with the two decisions that cannot be changed afterwards, which are where it goes and how deep it is. 13 min
- A goldfish pond through the year A calendar rather than an explanation. The pond guide covers what a pond is and the seasonal feeding page covers what to feed; this is the operational year, with the reasoning linked rather than repeated. 11 min
- Goldfish in a tub, trough or patio pond A container pond is a summer home, not a small pond. Done well, with the largest container you can manage, shade, air, very few fish and a plan for autumn, it gives goldfish a season of daylight, insects and algae that no tank can. 12 min
Plan it in numbers
Sizing tools
Equipment, researched
Buying guides
The best tanks for goldfish
Goldfish need floor space more than height and volume more than most shops admit. These are the tanks we would choose for one fancy, a group, or a pair of adult single-tails.The best filters for goldfish
Goldfish need more filtration than almost any other aquarium fish their size. These are the filters we would choose, and why, for tanks from 75 litres to indoor ponds.The best air pumps for goldfish aquariums
A goldfish tank can run short of oxygen, most of all when it is warm and crowded in summer. These are the air pumps we would pair with a sponge filter or air stone, sized by tank.Recommendations are researched, not tested by hand, and clearly separated from care advice. See our affiliate disclosure.
Cycling is covered in depth in the care section: cycling a goldfish aquarium.