When a fish has outgrown its tank
Some fish are sold small and grow far beyond the tank they were bought for. Spotting it early gives you choices.
Signs it has outgrown the tank
The fish cannot turn round without touching the glass, it swims against the same pane, or it is bullying everything else. Water that goes bad quickly after feeding is another sign: the load is too high for the volume.
Check the adult size on the fish page and run the real tank through the planner. If Space and Load are both Over, the problem is the fit, not your care.
Options before rehoming
A larger tank is the best answer when you can manage it. Otherwise, check whether lowering the stock around it would bring Load back into range, though that cannot fix a fish that has run out of length.
Rehoming it well
Contact a local fish club, a specialist aquarium shop or a rescue group. Be honest about the size and temperament of the fish. Some shops will take fish back, and others will refuse large predators, so ask first.
Do not put it in a pond or river. Many aquarium fish are invasive and will damage local wildlife, and in many places releasing them is illegal. A fish that is dying or cannot be placed is better ended humanely by a vet or an experienced keeper.
Avoiding it next time
Look up the adult length before buying. Every species page here shows it, with the tank length it needs.