New tank syndrome explained

What is new tank syndrome?

“New tank syndrome” is when fish get sick or die in a newly set-up aquarium because the tank isn’t cycled. There aren’t enough beneficial bacteria to turn fish waste (ammonia) into nitrite and then nitrate. Ammonia and nitrite build up and poison the fish.

How to prevent it

• Cycle before fish — Use fishless cycling so the tank processes ammonia and nitrite before any fish are added.
• Stock slowly — Add fish in small groups so bacteria can keep up. Avoid overstocking.
• Test regularly — During cycling and after adding fish, test ammonia, nitrite, and nitrate. Log results (e.g. in App-aquatic) to spot spikes early.
• Don’t overfeed — See how much to feed.

If you already have fish in an uncycled tank

Test immediately. If ammonia or nitrite is above zero, do a partial water change (30–50%) and reduce feeding. See ammonia spike and nitrite spike for step-by-step help.

Quick takeaways

• New tank syndrome = uncycled tank; ammonia and nitrite poison fish.
• Cycle before adding fish; stock slowly; test and log parameters.
• If fish are already in: water changes, reduce feeding, test until cycle is established.