Flashing (when guppies rub or “scratch” against objects) is often treated as a parasite problem immediately. Sometimes it is. But in many home tanks, flashing is the fish telling you they are irritated — and irritation can come from water chemistry swings, ammonia/nitrite traces, temperature stress, or even strong flow. Before you medicate, run a clear checklist. It saves money and prevents unnecessary stress on fish.
Step 1: Check ammonia and nitrite
If either is above 0, treat that as the primary issue. Do a water change, reduce feeding, increase aeration, and check your filter flow. Even low traces can irritate gills and skin.
Step 2: Check temperature and oxygen
Warm water + low oxygen can trigger restless behaviour. Increase surface agitation and confirm the tank is in a stable range (avoid sudden changes).
Step 3: Look for physical clues
- salt-grain spots: could be ich
- clamped fins + lethargy: often water stress
- rapid breathing: oxygen or gill irritation
- multiple fish affected quickly: often water-related
Step 4: Review recent changes
Most flashing outbreaks follow a trigger: a big water change, filter cleaning, new fish, new decor, or a temperature spike. If you can identify a trigger, you can often fix the issue without medication.
Step 5: Only then consider targeted treatment
If water is confirmed stable and flashing continues, then a targeted parasite plan makes sense. Keep oxygen high during any treatment and avoid stacking multiple medications at once unless you know exactly why.
Flashing is a symptom, not a diagnosis. A calm checklist approach fixes many cases in 24–48 hours and prevents unnecessary chemical stress in the tank.
