The short answer for a sparkling gourami
Start sparkling gouramis in at least a 10-gallon tank for a pair or small group, and choose a 15- or 20-gallon when the group includes several males. Sparkling gouramis are among the smallest gouramis, jewel-spangled fish that communicate with audible croaks during courtship and disputes. They are gentle by gourami standards, yet a male still claims a planted corner, builds a small bubble nest under a leaf or floating plant, and chases rivals from it. Enough footprint for two or three such corners, each screened by stems, is what turns croaking disputes into brief theatre instead of a tank-long feud.
Why still, planted water matters as much as volume
In the wild these fish live in slow, swampy, plant-choked water, and the home version of that is gentle filtration, preferably a sponge filter, floating plants dimming the light, and thick planting reaching the surface. Strong flow from a hang-on filter outflow exhausts them and destroys nests, so baffle or soften it. Their labyrinth organ lets them breathe surface air, and they make constant small trips up through the plants, which is another reason surface cover and easy access to the top belong in the plan. A tight lid keeps the air above the water warm and humid for those breathing trips and stops any jumper during a dispute.
Depth is the least useful dimension to add. A low, wide tank with layered planting gives every fish a level to occupy, while a tall narrow one stacks them into the same few corners.
Groups, nests, and peaceful tankmates
A group with more females than males spreads courtship attention and looks natural, a constant quiet shifting of fish through the stems with fins flaring and colours sparking blue and red in the right light. Feed small live, frozen, and fine crushed foods among the plants so subordinate fish eat out of the dominant male's sight. Companions should be small and unhurried: tiny rasboras, small peaceful bottom fish, and similar calm species. Nippy fish shred their trailing fins, and large fish simply end the croaking by making everyone hide.
When a male guards a nest, expect him to hold one corner firmly for a few days. That is normal and only becomes a housing problem if the tank offers the other fish nowhere beyond his reach, which loops back to footprint and planting rather than any flaw in the fish.
Reading a sparkling gourami tank that feels crowded
Constant croaking with chasing, a pale fish living at the surface in one spot, or nests repeatedly built and abandoned in high flow all suggest the layout, not the species, needs work. Add surface plants, break sightlines, and only then consider whether the group has outgrown the tank. Because the whole peace settlement rests on particular plant clumps staying in place, KinsLe Family Sharing records the planting map in the shared profile so a tidy-up never removes a male's only screened corner.