Most fish lost in a new tank die because it was never cycled. What the nitrogen cycle actually is, how to cycle a tank with or without fish, and how to tell — with a test kit, not a calendar — when it's safe to add livestock.
A new aquarium can look perfect — clear water, plants in place, filter humming — and still be unsafe for fish. What it's missing is invisible: a colony of bacteria that turns fish waste into something harmless. Building that colony is called cycling, and skipping it is the most common reason fish die in their first few weeks in a new tank.
Fish release ammonia through their gills and waste, and uneaten food and decaying leaves add more. Ammonia is toxic even at low levels. In an established tank, two groups of beneficial bacteria deal with it in two steps:
These bacteria don't float around in the water in any useful numbers. They colonise surfaces — above all your filter media, but also the substrate, hardscape and plant leaves. That's why the filter is the heart of the cycle, and why its media should never be rinsed under the tap.
Cycling without fish means feeding the bacteria an ammonia source yourself, so no animal has to live through the toxic spikes along the way.
Usually four to six weeks, sometimes longer. Bottled bacteria and seeded media shorten it, and warmer water speeds bacterial growth. Very soft, acidic water — around pH 6 or below — can slow the process noticeably, which is worth knowing if you're planning a blackwater setup.
If you've used an active aquasoil such as ADA Amazonia, expect it to release ammonia on its own for the first few weeks. That helps feed the cycle, but it also means frequent water changes in the first month — our substrate guide covers how to manage it.
If fish are already living in an uncycled tank, you can still get through it. The priority becomes protecting them while the bacteria catch up.
Every tank cycles at its own pace, so "wait two weeks" is only a guess. A liquid test kit for ammonia, nitrite and nitrate tells you exactly where you stand. We stock individual and all-in-one freshwater test kits — ammonia and nitrite are the two readings that matter most while cycling.
Usually four to six weeks without help. Bottled bacteria or used filter media from a healthy tank can shorten that, while cold or very acidic water can slow it. Test rather than count days — the tank is ready when it clears 2 ppm of ammonia to zero ammonia and zero nitrite within 24 hours.
Not safely. Without an established bacteria colony, ammonia and nitrite build up quickly and can kill fish within days. Cycle the tank first, or if fish are already in, keep stocking light and change water whenever ammonia or nitrite reaches 0.5 ppm.
They can speed cycling up noticeably, especially alongside a steady ammonia source. They don't replace testing — keep checking ammonia and nitrite until both hold at zero before adding fish.
A milky white haze in the first week or two is usually a bacterial bloom as the tank settles. It's generally harmless and clears on its own, so keep testing ammonia and nitrite rather than adding chemicals to clear it.
Understand the physics of high turn-over biological canisters & hardscaping in our next 2-day session.
