Fin rot is one of the most common guppy issues, and it’s also one of the most misunderstood. People often treat fin rot as if it’s a single contagious disease. In reality, fin rot is usually an opportunistic infection. The bacteria that cause it are often already present in most tanks — they become a problem when fins are damaged and the fish is stressed, or when water quality slips and healing can’t keep up.
Why fin rot starts
- Water quality drift: rising nitrate and organics weaken finnage.
- Physical damage: fin nipping, sharp decor, strong flow, or net damage.
- Stress stacking: crowding, poor ratios, temperature swings.
- New fish introduction: stress plus new bacteria strains can trigger problems.
Step-by-step recovery plan
- Test first: confirm ammonia and nitrite are 0, and nitrate is controlled.
- Stabilise routine: consistent water changes and lighter feeding for 1–2 weeks.
- Remove causes: fix sharp edges, calm fin nippers, reduce strong currents.
- Support healing: stable temperature and good oxygen makes regrowth faster.
When to medicate
If fins continue to erode despite stable water and removed stressors, or if you see redness, ulcers, or rapid progression, quarantine and targeted medication becomes more reasonable. Medication works best when the tank is stable — otherwise you’re treating symptoms while the cause remains.
What fin recovery looks like
Healed fins often grow back with a clear/transparent edge first, then colour returns later. That regrowth is a good sign. If you hold stability and remove nipping, fins can return surprisingly well over time.
Fin rot is a signal: the system needs stability and the fish needs a chance to heal. Most cases improve dramatically when you treat the cause, not just the fins.
