One of the most frustrating situations in guppy keeping is losing fish “one by one.” There’s no dramatic crash, no obvious white spots, and water looks fine — yet every week another fish disappears. It feels like bad luck. But this pattern usually points to a slow underlying stressor rather than random chance. The key is to look for trends: water stability, feeding load, social stress, and whether the tank has a hidden source of chronic irritation.
Common causes behind “one by one” losses
- chronic water stress: nitrates rising fast, low oxygen, or subtle pH swings
- overstocking: waste load slowly overwhelms the system
- bullying: weaker fish get pushed out of food and rest areas
- new additions without quarantine: low-level parasites weaken fish over time
What to check first
- ammonia and nitrite (must be 0)
- nitrate trend (how fast it rises between changes)
- temperature stability day and night
- social behaviour: who is being chased or excluded
Why “looks fine” can be misleading
Water can look clear while still being stressful. Dissolved organics, high nitrates, and oxygen limitation don’t always show visually. That’s why logs and consistent testing can reveal a slow drift that fish feel before you see it.
If you’re seeing one-by-one losses, don’t assume luck. Look for the slow pressure point, reduce it, and you’ll often stop the losses. Stable guppy tanks should be predictable, not a weekly mystery.
