The short answer
Stress and illness look very similar in fish, and telling them apart matters because the fixes differ. The useful rule is: stress usually shows as behaviour changes (hiding, clamped fins, faded colour, off food) without physical marks, while illness often adds visible signs like spots, ragged fins, bloating or sores. Since stress frequently comes first — and poor water causes both — the honest first move is always to check your water.
Signs that lean toward stress
If your fish is withdrawn, pale, clamped, or breathing fast but has no spots, wounds or bloating, stress is the likely explanation. Common triggers are poor water quality, a temperature swing, overcrowding, aggressive tankmates, or a recent move. Remove the trigger and the fish usually recovers within hours or days. For the full list of signs, see how do I know if my fish is stressed?
Signs that lean toward illness
Reach for the disease explanation when you see physical changes: white salt-like spots (see what is ich?), frayed or milky fins (fin rot), a bloated pineconed body (dropsy), or trouble staying upright (swim bladder). Even then, the underlying trigger is often poor water, so correcting conditions is step one before any treatment.
What to do either way
Whether it’s stress or sickness, the response starts the same: test and fix the water, stabilise the temperature, and reduce stressors. If clear physical symptoms appear, isolate the fish and research a proper treatment, consulting a vet or experienced fishkeeper before dosing. This is general guidance, not a diagnosis — when in doubt, get an experienced pair of eyes on the fish. See also how do I know if my fish is sick?