The short answer
A fish hugging the bottom is usually reacting to its environment. The common culprits are cold water, poor water quality, or stress — and for some species, sitting low is completely normal. As with almost any behaviour change, the first step is to test your water and check the temperature before assuming illness.
Rule out water and temperature first
Cool water slows a tropical fish right down, so confirm your heater is holding the correct range for your species. Then test for ammonia, nitrite and nitrate with a liquid test kit — elevated ammonia or nitrite makes fish lethargic and is common in tanks that aren’t fully cycled. If the numbers are off, do a water change with dechlorinated, temperature-matched water.
Stress, species and tankmates
Newly added fish often sit still and low while they adjust — good acclimation and plenty of cover help. Bottom-dwelling species such as corydoras, loaches and plecos genuinely belong down there, so that’s no cause for alarm. A single fish hiding on the bottom may also be avoiding an aggressive tankmate, so watch how the others behave around it.
When to look closer
If the water tests clean, the temperature is right, and an active fish still lies on the bottom with clamped fins, laboured breathing or a loss of appetite, keep watching for other symptoms — compare with how do I know if my fish is sick? and why is my fish not eating?. For a fish that’s clearly deteriorating, seek advice from an aquatic vet or an experienced fishkeeping community rather than guessing at a diagnosis.