The short answer for a fork-tail rainbowfish
A fork-tail rainbowfish does best starting in a 20-gallon long tank, with a 30-gallon long as the better choice for a full, confident school. The fish itself is slender and small, with a deeply forked tail edged in yellow and bright blue eyes, but judging size by body length alone misleads. In a short cube or tall tank the school bunches, turns constantly, and never settles into the loose, darting formation that makes this species look so good. Length gives them a run of open water, room to display, and space to scatter when a bolder male chases.
Why open swimming room matters more than gallons alone
Fork-tails come from clear, slow forest streams, yet in an aquarium they are surprisingly athletic midwater fish. Males flare their fins and circle each other through the day, females weave between plants, and the whole group shifts direction together. A long footprint lets that display happen parallel to the glass instead of into it. It also separates the open swimming lane from planted refuge, so a fish that loses a display contest can drop back into cover rather than being followed around a tiny loop. Tall tanks waste much of their volume above and below that busy middle layer.
A secure lid is part of the size answer, not an extra. Fork-tails startle upward when lights snap on or a net enters the water, and a small gap around filter cutouts is enough for a jump. Weigh down or mesh over those openings from the first day.
Setting up the 20- to 30-gallon footprint
Arrange the tank with a clear central channel and planting concentrated at the ends and back: fine-leaved stems, floating plants softening the light, and a dark background that shows off the blue eyes. Keep flow gentle to moderate, enough to move plants slightly without pushing the school to one corner. A sandy or fine gravel floor suits their stream home, though they spend little time near it. Leave the front glass uncluttered so you can count fish each morning, because a missing jumper is often first noticed as a gap in the school.
If you are choosing between two tanks of similar volume, measure the front-to-back and left-to-right footprint and pick the longer one every time. A longer tank also gives a heater, filter intake, and plant clumps room to sit apart, so maintenance does not require dismantling the whole layout each time.
How many fork-tails, and what too small looks like
Keep eight to ten as a sensible starting school, and more if the larger tank allows it without crowding tankmates. In undersized groups one male monopolises the display space and the others fade, hide, and lose condition. Suitable companions are other small, peaceful midwater fish and gentle bottom dwellers that leave the swimming lane free. Avoid fin-nipping species, because those extended forked tails are an easy target.
Signs the enclosure is too small include constant glass-surfing along one pane, males sparring in the same few inches all day, and fish that only feed when food drifts into the plants. Those behaviours call for more length or a larger school, not more decoration packed into the same box. When family members share water changes and holiday feeding for this display, KinsLe Family Sharing keeps the school size, lid warning, and routine in one shared profile everyone follows.