practice
Fishless cycling vs fish-in cycling
You have a new tank. Do you cycle it with fish in it or not?
Short answer
Fishless, unless you already have fish and no choice. Fish-in cycling means exposing animals to ammonia and nitrite for weeks, and the daily water changes it requires are more work than waiting.
Side by side
| Dimension | Fishless cycling | Fish-in cycling | Favours |
|---|---|---|---|
| Animals exposed to ammonia and nitrite | None | All of them | Fishless cycling |
| Duration | 3–8 weeks unaided | Similar, but with daily intervention | Fishless cycling |
| Work involved | Test and dose every few days | Test and water-change daily | Fishless cycling |
| Can dose ammonia to the target | Yes | No — the fish set the rate | Fishless cycling |
| Can be accelerated with mature media | Yes | Yes | Neither |
| Full stocking possible at the end | Yes — the filter is sized to a real load | No — stock still has to be added gradually | Fishless cycling |
Which to choose
Fishless cycling
Cycle fishless. It is the standard for good reason and it is less work, not more.
Fish-in cycling
Only if you already have fish — an emergency rehoming, a failed tank, a gift. Then: daily testing, daily water changes to keep ammonia and nitrite below 0.25 mg/L, and minimal feeding.