The celestial pearl danio, also sold as the galaxy rasbora, is a tiny fish described in 2006 from cool, weedy spring pools in Myanmar.
Males show pearl spots over a deep blue body with orange-red fins, making it a popular nano species.
Wild populations were quickly over-collected after discovery, so seek captive-bred stock, which is now widely available and hardier.
It is not difficult to keep but is shy and small, needing a thoughtfully planted tank rather than a busy community.
Housing
A group of six or more suits a planted 40-litre (10-gallon) nano, filtered and held at 20-24C; they prefer cooler water than most tropicals and often need no heater in a warm room.
Dense planting, fine substrate and gentle flow recreate the still, vegetated pools they inhabit.
They are timid and need plenty of cover to feel secure and display.
Avoid strong current and overly warm water, and choose a well-cycled, stable tank, since such small fish are sensitive to swinging parameters in small volumes.
Diet
These are micro-predators that pick tiny invertebrates from plants and substrate.
Offer fine micro-pellets, crushed flake and powdered foods as the base, supplemented generously with frozen or live cyclops, baby brine shrimp, daphnia and microworms.
Feed small amounts once or twice daily, ensuring food is small enough for their minute mouths.
Live and frozen foods bring out their best colour and breeding condition, so rotate them in regularly alongside the dry staple.
Health
Captive-bred celestials are reasonably hardy, but wild-caught fish arrive stressed and prone to parasites and rapid decline.
Ich and fin rot follow chilling or unstable water, and their tiny bodies show illness quickly.
Buy captive-bred stock, acclimatise slowly and quarantine new arrivals.
Keep water stable and on the cool side, perform weekly partial changes, and avoid crowding, as the small volumes of nano tanks punish overstocking and inconsistent maintenance.
Temperament
Peaceful but shy, celestials are not a true tight shoal; males display and spar gently among themselves while staying close to cover.
A larger group spreads out this sparring and encourages confident, colourful behaviour.
They suit calm nano setups with other small, gentle species and shrimp, but are easily bullied or outcompeted by larger, faster fish.
Keep them with equally peaceful tankmates so they feed and display without intimidation.
A good fit for
Planted nano and biotope aquariums
Cooler, unheated or low-heat setups
Keepers seeking a captive-bred species
Peaceful shrimp-friendly community tanks
Common mistakes to avoid
Buying over-collected wild-caught stock
Keeping them too warm or in strong flow
Housing with larger, faster tankmates
Sparse tanks leaving shy fish no cover
Is your enclosure big enough?
Minimum volume40 litres (11 US gallons)
CompanyCannot be kept alone. The minimum is 6, and the space above is for that group.
In a fish tank the patient is usually the water. Test before you treat — dosing a healthy tank for a disease it does not have makes things worse.
Go now
Any detectable ammonia or nitrite in a stocked tankLarge water change now with dechlorinated, temperature-matched water. Stop feeding. Test again in 12 hours. Both are directly toxic. How toxic the ammonia reading is depends on pH and temperature, so the same number can be harmless or lethal.
Now
Fish gasping at the surface, or crowding the filter outflowIncrease surface agitation immediately, then test the water. Either the oxygen has crashed or the gills are being chemically burned. Both are hours-scale problems.
Now
Sudden deaths, more than one fish in a dayTest ammonia, nitrite, nitrate and temperature before doing anything else. Do not add medication yet. Multiple simultaneous deaths point at the water or the equipment, not at a pathogen.
Now
Today
Flashing — repeatedly scraping against decor or substrateTest the water, then look closely for parasites. Irritation is either chemical or parasitic; the test tells you which before you medicate.
Today
White salt-grain spots over the body and finsBegin the appropriate treatment today and check the temperature. The parasite's life cycle is temperature-driven, and it multiplies with every cycle you wait.
Today
Soon
Clamped fins, hiding, or refusing food for more than two daysTest the water and review recent changes. Usually the earliest visible sign of a parameter drifting.
A few days
Never add a new fish to a tank that is not fully cycled, and never treat blind. A test kit is cheaper than the fish.
This is general guidance for a healthy adult, not a diagnosis. An exotics vet who knows your animal always overrides it — and the time to find one is before you need one.
What it really costs
Setup, monthly, first year and across a whole life, itemised and with the electricity worked out from the actual wattage. Published figures for the same animal disagree by a factor of four, so the calculator shows ranges rather than picking one.
Beginner
Beginner
Intermediate
Intermediate
Intermediate
Intermediate