One of the most frustrating guppy problems is the “one by one” loss pattern. The tank looks fine, there’s no obvious outbreak, and yet every week or two another fish declines. This pattern is almost always a sign of a chronic stressor rather than a sudden disease. The fish that are slightly weaker go first, then the next, until the system is corrected.
The most common causes
- Low-level ammonia or nitrite: even small readings above 0 are stressful over time.
- High nitrate and organics: not instantly lethal, but they reduce resilience.
- Oxygen limitation: warm water, overcrowding, weak surface agitation.
- Unstable temperature: day/night swings can stress livebearers.
- Stress stacking: bullying, poor ratios, or constant chasing.
How to diagnose the pattern
- test ammonia and nitrite repeatedly (not once)
- check temperature stability over a full day
- look at feeding: leftovers and cloudy water are clues
- observe social dynamics: who is being targeted?
How to stop slow losses
- Stabilise routine: consistent moderate water changes.
- Feed cleaner: smaller portions, less waste, and remove leftovers.
- Increase oxygen: add an airstone or improve surface movement.
- Reduce pressure: lower stocking or separate problem fish.
When quarantine matters
If losses started after adding new fish, quarantine becomes important because a low-grade parasite or bacterial problem can spread slowly. Still, even then, water stability remains the foundation — sick fish do worst in unstable tanks.
“One by one” losses are solvable. Treat it like a system problem, not a mystery curse. When you remove the chronic stressor, the losses stop and the tank becomes predictable again.
