What Causes Ammonia in a Fish Tank? Why the Reading Is High

Cardinal tetra in a freshwater aquarium

Fish produce ammonia as waste, uneaten food and decay add more, and filter bacteria remove it. A reading above zero usually means waste is outpacing those bacteria, from a new tank still cycling to ammonia in the tap water.

Why Do Fish Die in an Aquarium? The Water and Setup Causes to Check

Planted freshwater aquarium with swordtails

The usual causes are ammonia and nitrite in an uncycled tank, unconditioned tap water, temperature swings, low oxygen, too many fish added too fast, and sudden pH changes. Test the water first.