Cold water fish live at room temperature without a heater, unlike tropical species, so they’re a straightforward, lower cost entry point into fishkeeping and Birdsville, Sydney’s small animal and reptile specialist, keeps a wide range in stock, from classic goldfish to Australian native species. This pet Cold Water Fish care guide covers everything from choosing the right tank to keeping the water healthy long term, whether you’re setting up an indoor aquarium or an outdoor pond in the Sydney climate.

• Species covered: goldfish varieties, koi, rosy barbs, medakas, white clouds, and Australian natives such as rainbowfish
• Adult size: varies by species, from a few centimetres for smaller natives up to 60cm or more for koi
• Lifespan: commonly 10 to 15 years or more for goldfish, koi can live several decades with good care
• Diet: omnivore, flakes, pellets and frozen food
• Tank temperature: 18 to 23°C
• Outdoor pond depth: at least 60cm for year-round stability in Sydney’s climate
• Licence requirement: none
• Experience level: beginner-friendly

On This Page: Cold Water Fish as Pets, Essential Supplies, Housing, Environment, Diet & Feeding, Behaviour & Handling, Common Health Issues, Parasite Control, Health, Hygiene & Safety, Toxic & Dangerous Hazards, Sexing, Record Keeping, Holiday Care, Transportation, Compatibility with Other Animals, Outdoor Sydney Ponds, Frequently Asked Questions, Recommended Vets and Links

Cold water fish live at room temperature without a heater, unlike tropical species, so they’re a straightforward, lower cost entry point into fishkeeping. Most of what Birdsville stocks are community fish, meaning they naturally do well sharing a tank with other peaceful species rather than needing to be kept alone. Sydney’s mild climate also means many cold water species thrive outdoors in a pond year round, not just in an indoor aquarium, opening up a lot more options than in colder parts of the country.

Essential Supplies for Cold Water Fish

• Suitably sized tank: sized for the maximum your chosen species will reach, with room to spare for fish you might add later plus ornaments, hides and plants.
• Gravel: enough to cover the base 3 to 5cm deep, roughly an inch at minimum, so plants and ornaments can be anchored. Always rinse thoroughly before adding it.
• Filtration: choose a filter rated for your tank size or larger. If a filter lists a litres rating like 1000 or 2000, divide by 10 for a rough guide to the tank size in litres it suits.
• Lighting: brings out fish colour and supports plant and fish health, as well as making the tank easier to enjoy.
• Aeration or a bubbler: oxygenates the water, keeping it cleaner and the fish healthier, and can double as a nice visual feature.
• Gravel vacuum: makes routine cleaning far easier by suctioning waste and debris straight out of the gravel.
• Fish net: useful for moving decor around or catching a fish for a health check or a tank move.
• Thermometer: cold water fish don’t need a heater, but you still need to confirm the tank is holding steady in the 18 to 23°C range, especially through a Sydney summer.
• Decorations, plants and hides: live or artificial plants, rocks and ornaments give fish somewhere to shelter and explore, and help the tank feel more like a natural environment.
• Quarantine or hospital tank: a small separate tank for new arrivals or a sick fish, see Parasite Control below.
• Food: flakes, pellets or frozen food depending on the species. Ask our team which suits your fish.
• Water conditioner: removes and neutralises chlorine and metals from tap water. Straight tap water will burn fish gills and cause them to suffocate, so this is never optional.
• Biostart: seeds the filter with the beneficial bacteria a cold water tank needs to run the nitrogen cycle properly.
• Waste controller: helps break down leftover food and waste into beneficial bacteria, cutting down on cleaning and improving tank health.
• Aquarium salt: supports gill function and slime coat production, which helps fish fight off disease, and tops up electrolytes. Only ever use aquarium salt, never table salt, which will kill fish.
• pH test kit: pH varies noticeably across different parts of Sydney’s tap water supply, so testing takes the guesswork out of getting it right.

Everything on this list is stocked in store at Birdsville, along with a large range of glass cleaners, scrubbers and algae-eating fish and snails to help keep the tank in check.

What You May Consider: driftwood for the aquarium, either pre-soaked and already sunk (our most popular option, often sold with a slow-growing anubias plant attached) or dry driftwood that needs around 2 weeks of soaking to release its tannins and sink before it goes in the tank; and a foam underlay under any larger tank, since an uneven base can crack the glass once it’s full of water.

Housing

Where you place the tank matters as much as the tank itself. Pick a spot with power points nearby for the filter, light and bubbler, since that access can end up limiting how big a tank you can run, and never place an aquarium in a busy walkway where it’s likely to get knocked. Choose the tank size around the species and numbers you actually want to keep, including room to grow, since a larger adult fish or a growing collection will need more space than a tank sized for day one.

Set the tank up fully before adding any fish. Rinse every non-living item that will go in with tap water only, then add gravel, filters, bubblers, plants and ornaments before the water. Lay clean plastic over the gravel while filling to avoid disturbing it, add water conditioner once the tank is full, and only add biostart after the conditioner has had a chance to work, since tap water chlorine can kill the bacteria in biostart if it’s added too soon. It’s possible to buy everything, set up and add a small number of fish on the same day, but cycling the tank for a week or two first gives far better results.

Environment

Cold water fish do best with a stable tank temperature of 18 to 23°C, no heater required. Good lighting supports both fish health and plant growth, and steady aeration keeps oxygen levels where they need to be. Test pH periodically, since Sydney tap water varies noticeably by area and pH plays a big part in overall water quality.

Diet & Feeding

Feed cold water fish once or twice a day, a mix of flakes, pellets or frozen food suited to the species, only as much as they can finish within about 30 seconds each time. If you’re feeding twice, space the feeds out morning and evening instead of one after the other. Overfeeding causes far more health problems in captive fish than underfeeding does, so when in doubt, feed less. A newly arrived fish should go without food for the first 24 hours while it settles in from the stress of the move.

Behaviour & Handling

Have the tank fully set up, running and ideally cycled for at least two weeks before new fish arrive, with pH and temperature checked and sitting in range. When fish arrive in a bag, float the sealed bag in the tank for 10 to 15 minutes so the water temperature equalises gradually and avoids the shock of a sudden change. Use a net to move fish into the tank instead of tipping in the bag, and discard the bag and its water rather than adding it to the aquarium.

Common Health Issues

Ammonia and Nitrite Poisoning

Usually shows up in a tank that hasn’t finished cycling, or one that’s overstocked or overfed. Watch for gasping at the surface, lethargy or red, irritated gills, and test water parameters at the first sign of trouble.

Ich (White Spot Disease)

A parasite that shows up as small white spots across the body and fins, often with fish flicking or rubbing against gravel and decor. It spreads quickly between fish sharing a tank, so treat as soon as you spot it with an aquarium-safe ich treatment, available in store.

Fin Rot

Ragged, discoloured or shrinking fin edges, usually linked to poor water quality or stress. Improving water quality is the first step, alongside treatment if it doesn’t clear up on its own.

Swim Bladder Disorder

Most common in fancy goldfish varieties, shown by a fish struggling to stay upright or float normally. Overfeeding and constipation are common triggers, so adjusting feeding is often the first thing to try.

Any newly purchased fish should ideally spend 4 to 6 weeks in its own separate quarantine tank before joining an established aquarium or pond. Birdsville recommends this window for every new animal we sell, since it gives you time to confirm a fish is healthy before it has any chance to pass something on to an existing tank or pond.

Health, Hygiene & Safety

Every 24 Hours

Do a quick head count and eyeball every fish for condition, ideally at feeding time. If it takes longer than 30 seconds to clear the food, you’ve fed too much. Check that the filter flow, bubbler and lights are all working properly.

Every 2 Weeks

Check the glass for algae and give it a scrub if needed, and gently shake ornaments and plants to dislodge debris. Follow with a gravel vacuum and a water change of around 30%, adding water conditioner to the fresh water just before it goes in. The whole process usually takes about 10 minutes. Save the water you remove to rinse filter media and cartridges, keeping the beneficial bacteria alive instead of washing it all away, and inspect live plants for dead leaves or overgrowth to trim back.

Once a Month

Test pH, ammonia, nitrate and nitrite levels before doing your water change. If algae has been a recurring problem, test phosphate too, since algae thrives on high phosphate levels. Roughly every 4 weeks, replace some of the older filter media, keeping part of it back each time so the bacteria colony isn’t wiped out all at once.

Toxic & Dangerous Hazards

Table salt will kill fish, so only ever use aquarium salt. Untreated tap water contains chlorine and metals that burn fish gills and cause them to suffocate, so always treat it with water conditioner before it goes anywhere near the tank. Keep aerosols, air fresheners and household cleaning sprays well away from an open tank or pond, and never release pet fish into local creeks, rivers or waterways, since species like goldfish can become an environmental pest outside a home aquarium.

Holiday Care

A short trip away is generally fine, since cold water fish handle a few missed feeds far better than being overfed by an enthusiastic house sitter. For anything longer, arrange for someone to check the tank, top up water as needed and feed lightly, and leave clear instructions on exactly how much to feed, since overfeeding is the most common holiday care mistake. We also stock holiday feeders if you don’t have someone to feed your fish that they can eat while you are away.

H2: Transportation

Move fish in a securely sealed bag or container with some of their existing tank water, inside an insulated box or esky to keep the temperature stable. Keep the trip as short as possible and out of direct sun, and don’t leave fish sitting in a parked car while you run an errand, even briefly. Leave bag in the tank for 10 minutes for them to acclimate before releasing them into their new environment. We also supply the bad with the sale of any fish.

Outdoor Sydney Ponds

Sydney’s mild climate means many cold water species do just as well in an outdoor pond as in an indoor tank, and a pond gives fish far more natural swimming space. Summer heat is the real challenge here, not winter cold, so aim for a pond at least 60cm deep, since deeper water holds a steadier temperature. Some shade from pond plants or careful positioning out of all day sun helps prevent overheating, and good aeration matters even more in summer to keep oxygen levels up. Aquatic plants also help keep the water chemistry balanced naturally.

Goldfish varieties such as comets, sarasas and shubunkins, koi, rosy barbs, medakas and white clouds in ponds with good flow all suit outdoor life here, along with Australian natives including Murray rainbowfish, crimson spotted rainbowfish, southern blue eye and fire tail gudgeons, which also do a great job keeping mosquito numbers down. Not every cold water species is suited to pond life, so if you’re weighing up an outdoor setup, ask our team which of our fish will do best in your particular pond.

Frequently Asked Questions

What temperature should a cold water fish tank be?

Cold water fish do best kept at a stable 18 to 23°C, and unlike tropical fish, they don’t need a heater to get there in most Sydney homes. A sudden temperature swing is more of a risk than the actual number, so acclimate new fish slowly by floating the bag before releasing them.

How often should I feed my cold water fish?

Feed only as much as your fish can finish within about 30 seconds, once or twice a day. Overfeeding is a far more common problem than underfeeding, and leftover food fouls the water and drives up ammonia, so when unsure, feed a little less rather than more.

Can goldfish and other cold water fish live outdoors in Sydney?

Yes. Sydney’s climate suits many cold water species in an outdoor pond year round, provided it’s at least 60cm deep so the temperature stays stable through summer heat. Not every species suits pond life though, so check with our team before choosing fish for an outdoor setup.

How long do cold water fish live?

Cold water fish lifespan varies a lot by species: goldfish commonly live 10 to 15 years or more with good care, and koi can live for several decades in the right setup. That long lifespan is a good reason to get the tank or pond right from the start rather than cutting corners.

Do you always have cold water fish in stock?

Yes, cold water fish are one of our most consistently stocked categories, spanning goldfish varieties, koi, natives and other community species. Exactly what’s available shifts week to week, so pop in or call ahead if you’re after something specific for your tank or pond.

Is cold water fishkeeping a good hobby for beginners?

Yes. Cold water fish are considered one of the more forgiving starting points in the aquarium hobby, since they don’t need a heater and many species are hardy community fish. Setting the tank up properly and cycling it before adding fish matters more than any ongoing daily effort.

Visit Us

Head into Birdsville at 684 Botany Road, Alexandria NSW 2015, servicing the Eastern Suburbs and Inner South Sydney, to see our current cold water fish and get properly set up. Our team can help you choose the right tank, pond or supplies for exactly what you’re after. If you’re weighing up something different to a fish tank, our tarantula, scorpion and hermit crab care pages are a good next stop.

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