The short answer for a Norman's lampeye
Start a group of Norman's lampeyes in at least a 10-gallon tank, and choose a 20-gallon long when you can. The fish are small and slender, marked mainly by that striking half-moon of electric blue on the upper eye that catches light as they turn. They live in the top layer of the water, travelling as a loose school along the surface, so the useful dimension is the length of the surface run rather than the depth below it. A long, shallow-feeling tank serves them better than a tall one of the same volume.
Why the school, not the single fish, sets the minimum
One lampeye alone is a nervous fish. Their confidence comes from numbers: in a school of eight to twelve or more, individuals spread out, feed boldly at the surface, and show that eye-flash constantly as they signal to each other. In a trio they bunch under floating plants and vanish from view. Sizing the tank for the school you should actually keep, rather than the three fish a shop bag suggests, is the difference between a lively surface display and a tank that looks empty. The 20-gallon long also leaves the lower layers free for a few peaceful bottom fish without anyone competing for the same band of water.
They are accomplished jumpers, launching through gaps when startled or during evening activity. A full, tight lid with cutouts covered is a sizing requirement in the practical sense: the best tank fails if its school removes itself through a two-inch slot.
Laying out the surface layer they live in
Float plants across part of the surface for shade and security, but leave open lanes where the school can cruise and where food can be sprinkled. Gentle flow suits them; a strong filter outflow that pins them to one end undoes the benefit of the longer tank. Fine floating foods and small crushed items fed at the surface match their upturned mouths, and feeding in the open lane lets you count heads while they are distracted. Keep the water clean and stable, since small fish in a modest volume feel swings quickly, and do partial changes little and often rather than rare large ones.
Companions should be small and calm: tiny rasboras, small peaceful tetras, and quiet bottom dwellers work, while boisterous surface fish and anything large enough to regard a lampeye as food do not. Males display to females with fin flicks within the school, and a roomy group absorbs that attention without any one fish being chased relentlessly.
Reading a lampeye tank that is too small
School pressed permanently under the floaters, fish refusing to cross open water, or repeated losses found beside the tank all say the enclosure or its lid is failing them. So does a school that dwindles below six and goes quiet; topping numbers back up restores behaviour faster than rearranging decor. Given length, numbers, and a sealed top, Norman's lampeyes are an easy, sparkling surface school. Keeping the target school size, the no-gaps lid rule, and the surface feeding spot recorded through KinsLe Family Sharing means a helper topping up the school buys the right fish in the right numbers.