The short answer
A fish that suddenly stops eating is almost always telling you something about its environment, not its appetite. The usual causes are new-tank stress, a temperature that’s too low, or a dip in water quality. Before assuming illness, the first step for any behaviour change is the same: test your water for ammonia, nitrite, nitrate and check the temperature.
Test the water and check the temperature
Ammonia or nitrite above zero stresses fish and suppresses appetite fast, and it’s common in tanks that aren’t fully cycled. Grab a liquid test kit and check the numbers. If anything is off, do a water change with dechlorinated, temperature-matched water.
Temperature matters just as much. Tropical fish become sluggish and lose interest in food when the water is too cool, so confirm your heater is holding the right range — see what temperature a tropical aquarium should be.
Stress and new surroundings
Newly added fish very often refuse food for a day or two while they settle. That’s normal — give them cover, keep the lights low, and don’t crowd them. Careful acclimation reduces the shock. Bullying from tankmates, a fish that’s naturally shy, or the wrong food type (too large, or floating when the fish feeds low down) can all play a part too.
When to look closer
If the water tests clean, the temperature is right, and the fish still won’t eat after several days — or if it looks thin, listless or shows other symptoms — something more may be going on. Watch closely and compare against how do I know if my fish is sick?. For a worsening case, consult an aquatic vet or an experienced fishkeeping community rather than guessing at a treatment.