The short answer for a single pea puffer
A pea puffer is one of the smallest freshwater puffers, about the size of the fingernail it is named after, yet it needs more thoughtful space than its size suggests. One fish can live in a 5-gallon tank, but a 10-gallon aquarium is the better home: more water to keep stable, more floor to explore, and more room to build the dense, broken-up planting this curious hunter spends its day patrolling. Floor area matters more than height. A long, low tank gives a puffer a real territory to work; a tall, narrow one mostly gives it empty water it will never use.
Why a puffer measures space differently
Pea puffers do not school, drift, or share politely. A single fish investigates every leaf, hovers over the substrate hunting tiny snails and other small prey, and treats favoured plants and caves as its own property. In open, bare water that territorial instinct has nowhere to settle, and the fish often ends up glass-surfing or hiding. Broken sight lines are what turn gallons into usable space: thick plant clumps, driftwood, and small caves divide the tank into separate visual pockets, so the puffer can patrol from pocket to pocket and always has somewhere that feels like its own ground. More structure genuinely means more room for this species.
Laying out one puffer's tank
Start with a soft, fine substrate the fish can hover over and pick through, then plant heavily from back to front, leaving only small open patches for feeding and observation. Add a cave or two and some wood so the puffer can rest out of the light and stake out a sleeping spot. Keep the filter gentle but effective, because puffers are messy eaters for their size and leftover food fouls a small tank quickly. A secure lid matters too, since a startled puffer can jump during water changes or when a hand passes over the glass. Feed in the same open patch each time and the fish will learn to meet you there, which makes health checks far easier in all that planting. That meeting spot also reveals appetite changes early, because a puffer that stops arriving for its favourite foods is telling you something long before it looks unwell.
What changes if you keep a group
Groups are where space mistakes hurt. Several pea puffers together need a substantially larger, very densely planted tank so each fish can hold a separate territory mostly out of sight of the others, with more females than males and enough structure that a chased fish can break away and disappear. In a small or sparsely planted group tank, the boldest puffer claims everything and the rest fade, refuse food, and wear torn fins. Watch feeding closely: every fish should emerge, hunt, and eat at each meal. If one puffer is always missing, the layout, not the fish, is the problem, and the honest fix is more space and more cover rather than hoping they settle. Families who share puffer duties can use KinsLe's Family Sharing so the tank size, planting plan, and each fish's feeding notes stay in one profile everyone works from.