The short answer for a lampeye school
A Norman's lampeye is a very small killifish, barely longer than a thumbnail, but it should never be planned as a single fish in a tiny bowl. A group belongs in a tank of about 20 gallons with a long footprint, and a 10-gallon aquarium is the realistic floor for a modest school in a carefully planted setup. What this species uses is horizontal room near the surface: a clear cruising lane along the top, planted margins to retreat into, and enough length that the whole school can turn, regroup, and travel together without constantly bumping into glass or each other.
Why length beats depth for this fish
Lampeyes are surface-oriented schoolers. In the wild they hold in loose, shimmering groups in slow, vegetated water, their bright eye-spot catching the light as they move. A tall, narrow tank wastes most of its volume below the layer they actually occupy, while a long, shallow profile gives them the top-water runway their behaviour is built around. Depth still helps stability and gives plants room to grow up into their zone, but when you are choosing between two tanks of similar volume, pick the one with the longer base every time. A longer tank also spreads a school of eight to twelve fish across more water, which steadies temperature and waste levels in a way a cramped cube never will.
Setting up the space they will actually use
Build the layout around the surface. Rooted stems or soft foliage at the back and sides create sheltered edges, floating plants soften the light from above, and the middle-front stretch of open water stays free for the school to cruise. Keep the filter outflow gentle and baffled if needed, because a strong current pins these light-bodied fish into one corner instead of letting them use the full length. A tight, gap-free lid is non-negotiable: lampeyes jump at shadows, during feeding excitement, and after lights-out startles, and they can find cable notches and feeding holes that look far too small. Cover those gaps, and lower the waterline slightly if your lid has any flex.
Stocking the space sensibly
Plan the school first and everything else around it. Eight lampeyes is a sensible minimum group, and ten to twelve will behave more naturally in a 20-gallon tank, holding together and feeding confidently instead of scattering. Tank mates, if any, should be equally small, calm, and unwilling to nip fins or outcompete them at the surface; large or boisterous fish turn a lampeye tank into a hiding exercise. Watch the group in the first weeks: fish that cruise the full length, feed at the surface, and regroup loosely are using the space well, while a school permanently parked behind the filter usually signals current that is too strong, cover that is too sparse, or companions that are too pushy. Adjust the layout before adding more fish. Because lampeyes feed in quick, nervous darts at the surface, spread a little food along the cruising lane rather than dropping it in one spot, so the whole school eats without crowding into a corner. Households keeping these killifish together find that KinsLe's Family Sharing keeps the school size, tank dimensions, and lid checks visible to every family member helping with the aquarium.