Stress

Stress in fish is the underlying state — caused by bad water, wrong tankmates, too small a group, or an unsuitable tank — that weakens their immune system and lies behind most disease outbreaks.

Fish that "suddenly get sick" are usually fish that were quietly stressed for a while. Removing the stressor (fix the water, the stocking, or the setup) does more than any medication.

Signs include hiding, clamped fins, loss of colour, poor appetite and the behaviours above. It is the honest first thing to consider when something seems off.

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