The short answer
Flashing — rubbing or scraping the body against surfaces — means the skin or gills are irritated. The two big causes are water quality (or a parameter swing) and external parasites. Because these look alike, the first step is always to test your water before assuming a parasite and medicating.
Water quality comes first
Ammonia and nitrite chemically burn the skin and gills, and a sudden change in pH, temperature or a large unmatched water change can irritate a fish just as much. Test with a liquid test kit; if ammonia or nitrite is present, do a prompt water change with dechlorinated, temperature-matched water. Make sure new water is always matched to the tank — see how to do a water change. Uncycled tanks are a frequent trigger, so review how to cycle an aquarium.
Ruling out parasites
If the water tests clean and stays stable but flashing continues — especially alongside tiny white spots, a dusting sheen, clamped fins or increased mucus — an external parasite may be involved. This is where quarantine matters: new fish and plants are the usual way parasites arrive, which is why we recommend a quarantine step. If the gills look affected and the fish is also breathing hard, cross-check with why is my fish breathing fast?.
Getting it right
Don’t dose parasite medication blindly — treating clean water stresses fish for nothing, and the wrong medication can do harm. Confirm the environment first, observe carefully, and if flashing persists or worsens after the water checks out, describe the exact symptoms to an aquatic vet or an experienced fishkeeping community before choosing a treatment.