“Flashing” is when a guppy darts and rubs its body against objects as if it’s itchy. It looks dramatic, and it’s easy to assume parasites straight away. But flashing has multiple causes. The smartest approach is to work through a short checklist so you don’t medicate unnecessarily or miss the real issue. In guppy tanks, the most common early triggers are water irritation (ammonia/nitrite) and sudden changes.
Step 1: check the water first
Test ammonia and nitrite immediately. Any reading above 0 can irritate gills and skin. Also check nitrate and pH to see if the tank is drifting.
Step 2: look for pattern and frequency
- One flash after a water change: often mild irritation or temperature mismatch.
- Repeated flashes all day: more concerning, especially if multiple fish do it.
- One fish only: could be an individual issue, injury, or stress.
Step 3: observe other signs
- clamped fins, hiding, reduced appetite
- fast breathing or hanging near the surface
- visible spots or film (less common, but very telling)
Step 4: correct the environment
- increase aeration
- do a safe water change if toxins are present
- stabilise temperature and reduce stress (dim lights)
Step 5: consider parasites only after basics are stable
If ammonia/nitrite are 0, oxygen is good, and flashing persists across fish, parasites become more likely. Quarantine and targeted treatment makes sense then — but your first win is always stable water.
Flashing is a signal, not a diagnosis. If you respond in the right order, you solve most cases faster and avoid unnecessary chemical swings.
