The short answer
A goldfish parked on the bottom is usually a warning sign, and the three most common causes are poor water quality, a swim-bladder problem, or cold stress. Occasional resting can be normal, especially at night, but persistent bottom-sitting — often with clamped fins or no appetite — means it’s time to investigate. Start with a water test, because bad water is the single most common reason and the easiest to fix.
Check the water first
Ammonia and nitrite are toxic even in small amounts and leave fish lethargic and hugging the bottom. Test for ammonia, nitrite and nitrate; if any ammonia or nitrite shows, do a water change and check that your tank is cycled and your filter is working. Goldfish produce a lot of waste, so water quality slips faster than in a lightly stocked tropical tank.
Swim bladder and buoyancy
If the water’s clean, a swim-bladder issue is the next suspect — especially in fancies. A fish that struggles to lift off the bottom, or tips and sinks, may be constipated or gulping air with floating food. Try fasting for a day or two, then offer soaked, sinking food or a de-shelled, blanched pea. Switching to sinking pellets long-term helps prevent it. See why is my goldfish swimming sideways?
Temperature and stress
Goldfish are cold-water fish, but sudden temperature swings stress them and can drive them to the bottom. Keep conditions stable and avoid dumping in cold, unmatched water. If you’ve just done a big change or moved the tank, give the fish time to settle.
What to do next
Test, change water if needed, and watch for other symptoms. If the fish also has spots, fuzzy patches or clamped fins, read how to tell if a fish is sick. Keep a good test kit handy and strong filtration running for this messy fish.