The short answer for a pea puffer
Keep a single pea puffer in no less than a 5-gallon tank. For more than one, think in terms of territory rather than gallons per fish: a heavily planted 20-gallon long suits a small shoal, because each puffer claims a patch of planting to patrol and sleep in. Pea puffers are among the smallest freshwater puffers, barely an inch long, but they think like much larger fish. They hover, inspect everything with independently moving eyes, hunt pest snails through the plants, and remember exactly where food appears. A bare small tank bores and stresses them; a planted larger one keeps that busy mind occupied.
Why sightlines matter more than the tape measure
In a group, conflict is managed by breaking up what the fish can see. Dense stems, moss, wood, and caves interrupt the line of sight so a chased puffer disappears instead of being followed around a clear box. This is why a planted 20-gallon can hold a shoal peaceably while a sparse 10-gallon with three puffers turns into daily fin-nipping. Males are especially territorial as they mature, and a group with more females than males, all added together into a thicket of planting, settles best.
Sexing youngsters in a shop is difficult, which is another argument for starting with the larger planted tank: it forgives a less-than-perfect mix that a nano tank would punish within weeks.
Feeding and furnishing a puffer enclosure
Pea puffers are carnivores with beaks that keep growing. Small snails are both food and dental care, crunching the shell helping wear the beak, alongside bloodworm-type frozen foods and other meaty items offered with tweezers or in a feeding cone so you can confirm each fish ate. Many individuals refuse dry foods entirely, so a tank plan that depends on flake is a plan to starve them slowly. Plant thickly from the start, include floating cover, and keep filtration gentle but effective, because meaty feeding fouls a small volume quickly.
Tankmates are a gamble best avoided in the 5-gallon and approached cautiously even in the larger tank. Slow fish get their fins sampled, tiny shrimp get hunted, and fast community fish outcompete puffers at mealtimes. A species-only setup shows their behaviour at its best.
Signs a pea puffer needs a bigger, busier tank
Glass-surfing patrols on one pane, a subordinate fish living permanently behind the filter, bitten tail fins in a group, and food guarding all point to too little broken-up space. A lone puffer that begs at the glass and explores every plant is a puffer whose enclosure is working. Because their care hinges on who ate, where each fish sleeps, and keeping snail supplies coming, KinsLe Family Sharing lets everyone helping log feedings against the same shared puffer profile instead of double-feeding the boldest fish.