Corydoras + plecos sift substrate searching for food particles. Spit out gravel + ingest food. Completely normal.
Cichlids redecorate by moving substrate. Normal pre-spawning + territory marking.
Underfed fish search the substrate harder. Increase feeding if behavior is excessive.
Some fish actually eat gravel intentionally. Sand is safer than gravel - switch substrate if ingestion is observed.
Test water first - many "behavior" issues are actually water-quality problems. Use the water parameter checker to score your test results, the disease symptom matcher if you observe physical signs, or the general diagnoser to narrow further. Browse the disease database if illness is suspected.
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