Beneficial bacteria
This community is what separates a cycled tank from a lethal one. It is slow to establish, sized to the ammonia supply, and easily damaged by chlorinated water, medication and prolonged loss of oxygen.
In one sentence
The microbial community that performs nitrification — mostly ammonia-oxidising bacteria and archaea plus nitrite-oxidising Nitrospira — living as biofilm on filter media and every other surface.
Bottled bacteria products
Some products genuinely shorten cycling and some do very little. Two things separate them: whether the species included are the ones that actually dominate freshwater aquarium filters, and whether the organisms survived storage. Refrigerated products with a short shelf life are a better sign than shelf-stable ones with a long one.
Even an effective product does not make a tank instantly safe. Test rather than assuming, and stock gradually.
What damages the population
- Chlorinated tap water used to rinse media.
- Antibacterial medication dosed into the display tank — one reason to treat in a hospital tank where possible.
- A filter switched off for hours, which goes anoxic.
- Removing the fish for an extended period, which starves it.