Why he turned territorial
A male dwarf gourami in full color is a fish in breeding condition, and in the wild that means one job: hold a patch of calm surface, build a bubble nest in it and drive rivals and intruders away. In a community tank his patch is usually one planted corner under the light, and the tank mates he charges are not enemies he hates — they are fish that keep wandering into a nest site he is wired to defend. Chasing that happens mostly in one area, and flares when fish cross it, is territorial behavior. Chasing that follows one victim everywhere, all day, is bullying, and it needs a different response.
Break up the territory he is defending
The goal is to make his corner stop feeling like a defensible estate. Add tall stem plants or broad leaves that reach the surface in and around his patch, and let floating plants spread above it, so he cannot see the whole tank from his post and other fish can pass behind cover instead of crossing his open ground. A piece of wood or rock placed to interrupt the long sight line across the tank helps more than scattered ornaments do. Then rearrange the existing decor once — moving his landmarks resets the boundary he memorized and buys a calmer week while the new layout establishes.
Change the daily flashpoints
Most gourami charges happen at feeding, when every fish in the tank converges on one spot — which is, of course, his spot. Feed at two ends of the tank at the same moment, dropping his portion over his corner while the others eat at the far end, and let sinking food fall in a covered area for bottom fish so they are not chased off it. Watch which tank mates draw the attacks: slow, long-finned fish such as fancy guppies or a betta invite nipping from a territorial gourami, and small fish that constantly cross his corner may simply need the cover rearranged so their routes change.
When the mix itself is wrong
Two male dwarf gouramis in one tank is the commonest cause of relentless fighting, and no planting scheme fully fixes a genuine rival — one male should be rehomed or moved. Likewise, if one particular tank mate is hunted until it hides all day, stops feeding or develops torn and reddening fins, separate them the same day rather than waiting for the gourami to calm down; he usually will not while the trigger fish is present. A wound that looks fuzzy, white-edged or inflamed, or a victim that stops eating entirely, is the point to isolate the injured fish and contact an aquatics-savvy veterinarian, because injuries in stressed fish infect quickly.
Making the changes stick
Territory fixes fail when three people run three different experiments in the same week, so write the plan down where everyone follows it: with KinsLe's Care Teams, the two-spot feeding, the evening aggression check and any decor adjustment become assigned tasks marked complete with timestamps, and you can see from the record whether the calmer behavior followed the new layout or the separation is still needed.