Why ten gallons is the floor for guppies
Guppies stay small — males around an inch and a half, females closer to two inches and noticeably deeper-bodied — but the minimum is set by how they live, not by their length. They are active, social fish that should be kept in a group, and a group of six to eight adults plus the filtration and swimming room they need lands naturally at about ten gallons. A long ten-gallon also gives you a reasonably stable volume of water; in anything smaller, temperature and waste levels swing faster than a beginner can spot them, and guppies show stress quickly when conditions wobble from day to day.
The breeding factor that changes everything
The real sizing question is whether females are in the tank. Guppies are livebearers, and a female who has ever been kept with males can produce a batch of fry roughly every month for months, even after the males are removed. In a ten-gallon, one trio can become a crowded colony faster than new keepers expect, and crowding is what turns an easy fish into a maintenance problem: more waste, more competition at feeding time, and males pestering females without anywhere for the females to rest. If you want colour without population growth, keep an all-male group — the males are the showy sex anyway. If you do want to breed, one male to two or three females spreads his attention, and ten gallons is only a starting point for the first generation, not a home for the third.
Making a minimum-size tank work
Floor space matters more than depth, because guppies cruise the middle and top of the water where food arrives. A gentle filter is important — strong flow exhausts the long-finned fancy strains — and a close-fitting lid matters, since guppies will jump through a gap when startled. Plants, real or soft artificial ones, break up sight lines so a chased fish can drop out of view, and the fry that inevitably appear get hiding places among fine-leaved growth near the surface. Keep the stocking coherent at this size: guppies plus one small bottom-dwelling species is a workable plan, but every extra mid-water species eats into the margin the minimum was supposed to protect, and the tank stops being forgiving exactly when a beginner needs it to be.
When the honest answer is a bigger tank
Choose twenty gallons from the start if you plan to let a colony develop, want a community with other small fish, or know you will not be rehoming fry regularly. The extra water dilutes waste between water changes, gives males room to display without cornering females, and turns a borderline setup into a relaxed one that tolerates a busy week. Size down to the ten-gallon floor only if you are certain about an all-male group and disciplined about numbers when a pretty fish appears in the shop. However you stock them, recording the tank size, group mix and strain notes for each set of guppies in their KinsLe Pet Passport keeps the reasoning behind the setup attached to the fish themselves.