Celestial pearl danios, the tiny spotted fish also sold as galaxy rasboras, look like the perfect three-gallon desktop fish and are not. They are active, social micropredators with an involved social life: males colour up, display to each other, and spar over small patches of planted floor. Tank size for this species is really about giving that social life enough planted ground to play out without one male monopolising all of it.
Ten gallons as the honest starting point
A tank of about 10 gallons with a standard rectangular footprint houses a group of six to ten comfortably when it is well planted, and it gives males space to hold separate display spots. A 20-gallon long does the same job better for a larger group, spreading territories along its length and making the fish bolder in open water. Very small tanks compress the group until subordinate males spend their lives hiding, pale and stressed, behind the filter.
Depth is the least useful dimension. These fish live in the lower two thirds among plants, so a long, low tank of modest volume beats a tall cube of the same gallons. Whatever size you choose, fit a close lid, because a startled fish this small finds gaps you would swear were sealed.
Planting is what makes the gallons work
In the wild these danios live in shallow, plant-choked pools, and a tank planted like a meadow is the functional equivalent of a bigger aquarium. Moss clumps, fine-leaved stems, and low bushy plants break the floor into small visual territories, scatter light, and catch the eggs the fish scatter as they spawn in the planting. Leave small open patches of substrate between plant groups where males can display, and keep one gentle feeding area clear at the front so you can watch the whole group gather.
Floating plants in moderation calm them and bring them higher in the water, but keep growth trimmed so light reaches the meadow below. A dark substrate sets off the pearly spotting and encourages the fish to stay out in view rather than hugging the shadows.
Numbers, neighbours, and daily proof
Keep more females than males where the shop allows choosing, and stock the group in one go so territories form across the whole floor at once. Tankmates should be equally small and peaceful, or the danios will feed last and fade socially even in adequate space; many keepers give them a species-only tank for exactly this reason. Your daily check is colour: confident males are deep-bodied, orange-finned, and out displaying, while a tank that is too small or too bare shows you mostly females and hiding fish.
Feed the meadow, not just the fish: fine foods dusted into the planting let celestial pearls hunt particle by particle through the moss as they would in their home pools, and a group that forages for an hour after feeding uses every planted inch you gave it.
As the group settles, resist shrinking their planting during tidy-ups, since the cover is load-bearing in a way it is not for bolder fish. Saving the tank footprint, group mix, and meadow layout in KinsLe Family Sharing ensures anyone trimming plants at home leaves the territories and egg-catching moss in place.