water quality
New tank syndrome (uncycled tank)
Recurring or persistent fish illness in a tank less than 6–8 weeks old, almost always caused by the absence of an established nitrogen cycle. Beneficial bacteria that convert ammonia to nitrite and then to nitrate have not yet colonized the filter. Fish suffer from toxic ammonia and nitrite spikes.
Do first
- Test ammonia and nitrite immediately.
- Perform a 30–50% water change with dechlorinated water matched to tank temperature.
- Dose a dechlorinator that detoxifies ammonia and nitrite (look for one containing sodium thiosulfate + a colloid/slime coat component).
- Stop feeding for 24 hours.
- Add bottled beneficial bacteria to the filter immediately.
Escalate if
- Fish dying despite daily water changes reduce stocking immediately.
- Ammonia above 4 ppm emergency large water change required.
- After 12 weeks the tank still has detectable ammonia or nitrite: investigate filter capacity, bioload, or products interfering with nitrification.
Water clues
These readings can push this pattern higher or lower in the triage result.
ammonia above zero+10
Any detectable ammonia in a tank under 8 weeks old strongly confirms new tank syndrome.
nitrite above zero+10
Nitrite above zero alongside ammonia equals an actively cycling tank fish are at serious risk.
Care protocol
Follow only the steps that fit your species, tank inhabitants, and medication label.
Managing a fish-in cycle
- Test ammonia and nitrite daily keep a log.
- Perform water changes of 25–30% whenever ammonia exceeds 0.5 ppm or nitrite exceeds 0.25 ppm.
- Dose bottled beneficial bacteria (Nitrospira-containing products preferred) regularly.
- Add seeded filter media from an established tank if available. This is the fastest way to cycle.
- Reduce stocking to the absolute minimum while cycling.
- The cycle is complete when ammonia and nitrite both read zero for 7 consecutive days and nitrate is detectable.
Cautions
- Do not use ammonia-removing zeolite as it starves the developing bacterial colony.
- Do not clean the filter media during a fish-in cycle it houses the developing bacteria.
- Do not add more fish until the cycle is fully established.
Preventing new tank syndrome (fishless cycling)
- Cycle the tank without fish using pure ammonia (2–4 ppm) as the nitrogen source.
- Add bottled bacteria and dose ammonia every 2–3 days.
- Monitor ammonia and nitrite daily.
- When both ammonia and nitrite drop to zero within 24 hours of adding a dose of ammonia, the tank is cycled.
- Do a large water change before adding fish to remove accumulated nitrate.
- Add fish gradually do not fully stock on day one.
Cautions
- Fishless cycling takes 4–8 weeks patience is essential.
- Not all bottled bacteria products are equally effective Nitrospira species are preferred over Nitrobacter.
Source notes
References and context notes used for this triage entry.