Bottom-sitting means very different things for a corydoras and a normally active mid-water fish. Start with the species, then check the environment.
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Some fish belong near the bottom. Corydoras search the substrate, loaches shelter low, and plecos spend long periods attached to hard surfaces. Concern starts when a fish changes from its normal position or combines bottom-sitting with other symptoms.
Test ammonia and nitrite first because either can be dangerous in water that looks perfectly clear. Check nitrate and pH for context, then verify temperature with a reliable reading. Water that is too cool can slow a fish down, while overly warm water holds less dissolved oxygen and raises metabolic demand.
A new arrival may hide low after transport and acclimation. Give it cover, subdued activity around the tank, and time to settle while still monitoring the basics. Stress alone can explain temporary withdrawal, but it should not be used to dismiss worsening breathing or balance problems.
Look for rapid gill movement, clamped fins, loss of appetite, spots, fading colour, erratic swimming, or separation from a social group. A cluster of signs is more informative than bottom position alone. Record what changed recently, including maintenance, feeding, livestock, temperature, and equipment.
Check the filter outlet and surface movement. Reduced flow from a clogged intake or stalled air pump can lower oxygen exchange. If water and equipment checks point toward oxygenation, supplemental aeration can help while the cause is corrected; it is not a substitute for resolving ammonia, heat, or disease.
Avoid choosing medication from one vague symptom. Testing and observation narrow the problem without adding unnecessary treatment stress. The smallest safe response is usually to stabilize the environment first, keep notes, and escalate only when the evidence supports it.
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