“Flashing” is when guppies rub their body against objects, dart suddenly, or scratch along the substrate. It can look like a clear sign of parasites, but flashing is not specific. Fish flash when their skin or gills are irritated — and that irritation can be caused by poor water quality, sudden temperature swings, chemical residues, or parasites. The key is to follow a safe order of operations so you don’t medicate blindly and create a bigger problem.
Step 1: test water immediately
Start with ammonia, nitrite, nitrate, and pH. Even small ammonia or nitrite levels can irritate gills and trigger flashing. If anything is off, fix water first.
Step 2: stabilise environment
- do a moderate water change if tests show issues
- increase aeration (gills under stress need oxygen)
- check temperature for sudden swings
- remove potential irritants (sprays, soaps, residue)
Step 3: observe patterns
If flashing stops after water stabilises, it was likely environmental. If flashing continues, becomes frequent, or other signs appear (clamped fins, heavy breathing, white spots), parasites become more likely.
Step 4: treat deliberately
Only treat once you’ve ruled out obvious water causes. Targeted treatment is safer than stacking multiple medications. In a guppy tank, stability often resolves mild irritation without medication.
Flashing is a useful early warning sign. When you respond by testing and stabilising first, you protect fish and avoid unnecessary chemical swings that can make problems worse.
