Cycle your tank before you add fish

The few weeks of waiting that decide whether your fish live.

Fish produce ammonia, and ammonia burns their gills. In an established tank, bacteria living in the filter turn ammonia into nitrite and then into nitrate, which is far less harmful. A new tank has none of those bacteria. Growing them is called cycling.

How to cycle without fish

1. Set up the tank with dechlorinated water and leave the filter and heater running.
2. Add bottled ammonia until a test kit reads about 2 ppm.
3. Test every couple of days. Ammonia falls as nitrite rises, then nitrite falls as nitrate rises.
4. Whenever ammonia reaches zero, top it back up to 2 ppm.
5. The tank is cycled when it turns 2 ppm of ammonia into zero ammonia and zero nitrite within 24 hours.

This usually takes four to six weeks. Filter media or gravel from a healthy, established tank can shorten it a lot.

Before the first fish

Change most of the water to bring nitrate down, then add your first group. Keep testing for the first few weeks, because each new group raises the load on the filter.