Ammonia poisoning

Signs

• Often no clear outward sign: ammonia is colourless and odourless, so only a water test shows it
• Gill and tissue damage, stress, fish stop eating
• Abnormal (neurological) behaviour
• More bacterial infections and deaths, sometimes weeks later

Cause

Ammonia (the main waste product of fish, plus decaying uneaten food and organic matter) builds up faster than the biofilter can convert it: a new tank stocked before the filter is mature (new tank syndrome, usually 1-3 weeks after set-up), overfeeding, or a biofilter killed by a pH crash in a tank with no buffering left (old tank syndrome). Some tap or well water already contains ammonia.

What to do

Test ammonia (plus pH and temperature). Stop or reduce feeding for 1-3 days; do a 25-50% water change with ammonia-free water (Merck: at least 50% when total ammonia is 2 mg/L or more in a new tank); ammonia-binding products help short-term; keep oxygen up. EXCEPTION: in an old tank with a pH crash (often below 6), do not just do a big water change, because raising the pH makes the ammonia more toxic; move the fish and rebuild the system.

If a fish is badly ill, talk to an aquatic vet.

Prevention

Establish (cycle) the biofilter before stocking - a new biofilter needs about 6-8 weeks; don't overfeed; test ammonia about weekly and whenever fish behave oddly; do real water changes (remove old water) to keep alkalinity/pH stable.