Broadheath Bettas
Recommended Cold Water FishCare
A step by step guide
Welcome!
Coldwater species such as goldfish, white cloud mountain minnows and certain danios offer a different kind of aquarium from a tropical setup, thriving in cooler water. With good care, goldfish in particular can live 10–20+ years.
A note before you start: these are not one interchangeable group. Goldfish, white clouds and danios differ in adult size, ideal temperature, sociability and how much waste they produce. The setup principles below are shared; where they differ, we’ve said so.
a) Setting up your tank
Coldwater fish, especially goldfish, are active and produce a lot of waste, so prioritise space and filtration. Fully cycle the tank before adding fish. See our Tank Cycling Guide
What You’ll Need
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An aquarium sized for the adult fish and the number you intend to keep
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A suitably sized filter
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A thermometer
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A reliable liquid water test kit
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Water conditioner/dechlorinator
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A siphon and dedicated bucket for water changes
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Suitable substrate, plants and safe décor
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Aquarium lighting
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Appropriate food for the species
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Additional aeration if needed
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A plan for keeping the aquarium cool during very warm weather
Tank Size
Goldfish (fancy): allow at least 100 litres for the first adult fish and roughly 50 litres for each additional adult. A small group therefore needs a substantially larger tank than beginners expect. 150 litres is a reasonable practical starting point for two smaller fancy goldfish, but be ready to upgrade as they grow. Single-tailed (common/comet) goldfish grow larger and faster and need considerably more — they are pond fish in the making.
White cloud minnows / small danios: these need much less space than goldfish, but they are active shoaling fish and still need adequate swimming length. For white cloud mountain minnows and zebra danios, an aquarium around 60 cm long is a useful minimum starting point, with at least five fish kept together. Other danio species vary, so check the individual species before buying.
Long, low tanks with a generous surface area are ideal for all of them.
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Temperature
Coldwater fish prefer cooler conditions, but the best range depends on the species.
Goldfish: for indoor fancy goldfish, around 15–23°C is a useful range. Avoid prolonged overheating and sudden temperature changes.
White cloud mountain minnows: around 18–22°C.
Zebra danios: around 18–24°C. Other danio species vary, so check the species.
A heater is not normally needed in most UK homes. Monitor the actual water temperature during hot weather and use suitable cooling measures if necessary. Avoid sudden temperature changes, which can stress fish.
Plants and Enrichment
Live plants such as java fern, anubias and hornwort provide cover and structure and can contribute to nutrient uptake. Goldfish may graze on or uproot softer plants.
Lighting
Moderate with an 8–12 hour day/night cycle.
Filtration
All coldwater aquariums need an effective filter. For goldfish, use a generously sized internal or external filter with sponge or pads to trap solid waste, plus biological filter media where the beneficial bacteria that process ammonia and nitrite can grow. As a practical starting point, choose a filter rated to circulate at least around 4–5 times the aquarium volume per hour. Goldfish produce a substantial amount of waste, so sizing up is usually preferable.
Fancy goldfish do not appreciate being pushed around by a strong jet. If the outlet creates too much current, spread the flow with a spray bar or direct it against the glass.
For smaller white cloud or danio aquariums, a suitably sized internal or mature sponge filter can work well. Whatever filter you use, the real test is that it keeps the water clear of excessive waste and the aquarium consistently at 0 ammonia and 0 nitrite.
Substrate and Decor
Fine sand or smooth gravel. Avoid sharp-edged substrate or décor.
b) Water Quality
Good water quality is essential for goldfish and other cold-water fish. Test at least weekly, and more often in a new aquarium or if anything seems wrong.
Keep the water well oxygenated
Maintain good circulation and surface movement so the water can exchange gases efficiently. An air stone can help by increasing surface movement, particularly during warm weather or if oxygen levels may be low.
Water Changes
Carry out a partial water change of around 25–30% each week using dechlorinated water at a similar temperature. Larger or more frequent changes may be needed where stocking, feeding or test results show they are necessary.
Good circulation & gas exchange
6.5-8.0
0 mg/L
Species Specific
No more than ~20 mg/L
Above your tapwater level
Ammonia & Nitrite
Temperature
Nitrate
Feeding
Feeding
Goldfish are opportunistic omnivores and do best on a varied diet based around a good-quality food formulated specifically for goldfish.
Staple food: Use a good-quality goldfish pellet, granule or flake. Slow-sinking food can be particularly useful for some fancy goldfish. Some individuals swallow air when repeatedly feeding at the surface, which can contribute to temporary buoyancy problems. Buoyancy problems have many other possible causes, however, so do not assume that feeding is responsible if a fish begins floating abnormally.
Variety
Supplement the staple diet with small amounts of foods such as daphnia, brine shrimp or bloodworm, together with suitable vegetable matter such as blanched lettuce, shelled peas or courgette. These provide dietary variety rather than acting as treatments for digestive or buoyancy problems.
How much: For adult goldfish, a small feed once daily is a useful starting point. Younger growing fish may need smaller meals more often. Only offer what the fish can comfortably eat within a minute or two, and remove uneaten food. Adjust the amount according to the fish's size and body condition. Overfeeding also increases the amount of waste the filter has to deal with.
White cloud mountain minnows and danios: Feed a good-quality flake or small granule as the staple food, with occasional small live or frozen foods such as daphnia or small bloodworm. Feed only what the group can eat within a few minutes.
Settling your coldwater fish in after delivery
Your tank must be fully cycled and tested before your fish arrives.
1. Check the fish before opening the bag
Keep the sealed bag upright and away from bright light while you check the fish. If anything concerns you, take a clear photograph or short video while the bag is still sealed and contact us promptly.
2. Match the temperature if needed
If the bag and aquarium differ noticeably in temperature, float the sealed bag for around 10–15 minutes to reduce the difference. This is only to adjust temperature. Skip this step if they are already close.
3. Transfer promptly
Gently move the fish into the aquarium using a suitable soft net or clean container. Never pour the transport water into the aquarium. Discard it.
4. Let the fish settle
Keep the lighting low and minimise disturbance at first. Once the fish is swimming normally and appears settled, offer a small amount of appropriate food. Do not worry if it does not feed straight away.
5. Keep a close eye on the first week
Check the fish each day and test ammonia and nitrite regularly during the first week. Both should remain at zero. If either is detected, carry out a partial water change with dechlorinated water at a similar temperature and investigate the cause.
For more about what to check when your fish arrives and why we favour prompt transfer after courier delivery, see our Arrival and Your First Week guide.
Tank Mates
Do not assume that fish sold as “coldwater” or suitable for an unheated aquarium will automatically live well together. Check that their temperature range, adult size, activity level and behaviour are compatible.
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Goldfish: Fancy goldfish are generally best kept with other compatible fancy goldfish of a similar size and swimming ability. Avoid mixing fast single-tailed varieties such as commons and comets with slower fancy goldfish, as the faster fish can outcompete them for food. Adult goldfish may also eat fish small enough to fit in their mouths.
White cloud mountain minnows and small danios: Keep them in suitable shoals, ideally at least five, with more where space allows. They can live with other peaceful fish that share their temperature requirements, but remember that danios are very active and may unsettle timid species. Do not keep white cloud mountain minnows with goldfish large enough to eat them.
Other companions: Check the actual requirements of the species rather than relying on the labels “coldwater” or “tropical”. Some species have overlapping temperature ranges and can be compatible, while others do not. Shrimp may suit some unheated community aquariums, but small shrimp and shrimplets may be eaten by fish.
Regular Health Checks & Common Issues
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Observe
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Watch for changes in swimming, appetite, breathing, buoyancy or behaviour, as well as fin damage, white spots, sores or other new marks on the body.
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​Maintain excellent water quality, avoid overcrowding, and feed a varied diet. Many issues are prevented by generous filtration and regular maintenance.
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Common Issues
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Buoyancy problems: These can have several causes, including feeding, water quality, temperature, infection and the fish’s body shape. Check the water and review feeding first. If the problem persists, or the fish appears otherwise unwell, seek appropriate advice rather than treating it as a simple digestive problem.
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Fin damage, white spots or other signs of illness: Test ammonia and nitrite immediately and check the temperature, filtration and general aquarium conditions. Good water quality is important, but it does not replace diagnosis or treatment where disease or injury is involved.
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If illness or injury is suspected: Contact us if you would like help reviewing the husbandry. For diagnosis or treatment, seek advice from a veterinary surgeon with appropriate aquatic experience rather than guessing at medication.
Quarantine new fish before introducing them to an established aquarium where practical.
Mistakes to Avoid
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Choosing a tank that is too small: Goldfish grow substantially and produce a lot of waste. Inadequate space makes maintaining good water quality more difficult and restricts normal swimming and behaviour.
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Overfeeding: Excess food increases waste and can quickly damage water quality. Feed according to the fish’s size and condition and remove uneaten food.
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Assuming coldwater fish do not need regular maintenance: They still need a fully cycled aquarium, effective filtration, regular testing and routine water changes.
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Allowing the aquarium to become too warm: Monitor the actual water temperature during hot weather and take appropriate cooling measures if necessary.
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Choosing tank mates by the label “coldwater” or “tropical” alone: Check the temperature range, adult size, behaviour and activity level of the particular species.
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Treating unusual swimming or buoyancy as automatically being a feeding problem: Check water quality and the fish’s overall condition first. Persistent or severe problems need appropriate advice.