This page is an extension of the article Aquarium Nitrogen Cycle. The expectation is that you have read the article and understand the Nitrogen cycle beforehand.
What is "Fish-in" Cycling?
"Fish-in" Cycling refers to the act of using fish as the ammonia source to establish the beneficial bacteria (Nitrosomonas and Nitrobacter) in the aquarium. You may have been referred to this page as you have not completed your cycle. In which case, please ensure you have read Aquarium Nitrogen Cycle first.
Why is it dangerous?
In a healthy aquarium, ammonia is converted into nitrates faster than the waste can be produced and the ammonia built up. When a cycle is not properly established, the waste and ammonia builds up, which can cause a lot of problems;[1] ammonia burn, stress lines, changes in behaviour and not eating. Higher concentrations can injure, stunt and kill fish and plant life.
Exhaust all other options first
Fishwiki only recommends the use of fish-in cycles when all other options are exhausted.
Fish-in cycles are an unnecessary risk.
Before completing a fish in cycle, check to see if there are other options available such as:
- Move the fish into a cycled tank if safe to do so
- Temporarily rehome the fish
- Return the fish to the store until your cycle is ready
a fish-in cycle can be unpredictable and damaging to your livestock, so it should be avoided where possible. If these options are no longer available to you, you can opt to complete a fish-in cycle.
Note: You will require a test kit to successfully complete a fish-in cycle.
Step by step: Completing a Fish-In Cycle
Everyday, at the same time:
Test the tanks Ammonia, Nitrite and Nitrate levels. Compare them to the chart below. even if only 1/3 of the tests is high, complete the water change regardless as indicated in the corresponding box.
| test level | 0 ppm | 0.25-1 ppm | 1ppm + |
| Ammonia | no water change is needed | 20% water change daily until 0 ppm | 25%-50% water change, twice daily until 0 ppm |
| test level | 0 ppm | 0.25-1 ppm | 1 pmm |
|---|---|---|---|
| Nitrite | no water change is needed | 25% water change daily until 0ppm | 25%-50% water change, twice daily until 0ppm |
| test level | 0-20 ppm | 20-100 | 100+ |
|---|---|---|---|
| Nitrate | no water change is needed | 20% water change weekly until below 20 ppm | 25% water change, twice daily until below 100 ppm |
Tips for success
- Ensure all water changes are done with dechlorinated water. DO NOT overdose the conditioner. It will kill your fish.[2]
- Test daily until you see ammonia plateau at 0
- observe fish for signs of stress or injury
- feed less, do not over feed
- DO NOT change filter, substrate, or complete a 100% water change
- make sure tank is oxygenated and heated appropriately
Other Info
Plants can be very beneficial for fish-in cycles. Most plants have the ability to consume ammonia and nitrite, not just nitrates.[3] Thus, it makes sense to utilize plants to reduce the toxic levels and help keep levels low while the tank establishes a bacterial colony.