TankSnap › Guides › New tank water parameters
Getting started
New tank water parameters: what to test and when
A new aquarium is not ready for fish the day you fill it. The water has to grow the bacteria that make it safe. Testing is how you know when that has happened.
Why a new tank is risky
Fish waste turns into ammonia, which is toxic. Beneficial bacteria convert it, first into nitrite and then into nitrate, but a brand-new tank has almost none of them. Building that population is called cycling. Until it is done, ammonia and nitrite can build up to harmful levels. Read ammonia, nitrite and nitrate for the full picture.
Two ways to cycle
Fishless cycle (safest for the fish)
You add a source of ammonia and let the bacteria grow while the tank is empty. No animal is exposed to the spikes. The easiest source to control is aquarium-safe ammonium chloride (sold for cycling): you dose a known amount and check it with your test. Fish food also works, but it releases ammonia unevenly as it rots, so the numbers are harder to steer. Avoid household ammonia, because many products contain detergents or fragrance and the amount is hard to dose. Aim for about 2 ppm on your test, and follow the product's instructions.
If fish are already in the tank
Cycling with fish in is generally discouraged, because they are exposed to ammonia and nitrite while the bacteria grow, and that can injure or kill them. Choose a fishless cycle if you can. If fish are already in the tank, keep the stock very light, feed sparingly and test ammonia and nitrite every day. Do a water change whenever either one reads above zero, even if that means every day, and add aeration. Do not add more fish until both stay at zero.
Bottled bacteria products can shorten the process, but they do not remove the need to test. Check the numbers before you trust the label.
What a cycle looks like
Cycling typically takes a few weeks. Four to eight is common, but it varies with temperature, filter media and whether you seed it with media from an established tank.
- Ammonia appears and rises. Nothing is processing it yet.
- Nitrite appears as the first bacteria establish, while ammonia starts to fall.
- Nitrate appears as the second group arrives, and nitrite begins to fall.
- Ammonia and nitrite both read zero, and nitrate is present. Zero on its own does not prove the cycle is finished. The dose test below does.
How to know it is finished
The dose test is the real finish line. In a fishless cycle, add ammonia up to about 2 ppm. If ammonia and nitrite both read zero again within 24 hours, with nitrate showing, the bacteria can handle a fish load. Zeros taken before dosing do not prove it, because there may simply be no ammonia to process. Then do a large water change to bring nitrate down before adding fish.
If the cycle seems stuck
When nothing has moved for a week or more, one of these is usually the cause:
- Low pH or KH. The bacteria use up carbonate hardness as they work, so in soft water the pH can fall during a cycle, and they slow down as it drops, especially below about 6.5. Test pH and KH more often if your KH is low.
- Too little ammonia. The bacteria need food. If ammonia reached zero before nitrite showed up, dose again.
- Chlorine or chloramine. Untreated tap water, or a filter rinsed under the tap, can kill the bacteria.
- Too little oxygen. The bacteria need oxygen. Keep the filter running and the water surface moving.
- Temperature. Cold water slows growth, and very hot water can harm it. The mid-20s °C suit most tropical set-ups.
- Too much ammonia or nitrite. Very high levels can slow the process. If readings are off the scale of your kit, do a water change and dose less.
What to test, in order of importance
| Parameter | Why it matters | When |
|---|---|---|
| Ammonia | The first toxic product. Shows whether bacteria are keeping up. | Every day or two while cycling |
| Nitrite | Toxic middle step. Its fall to zero signals the cycle is closing. | Every day or two while cycling |
| Nitrate | Confirms the cycle is working and tells you when to change water. | Once or twice a week |
| pH | Affects how toxic ammonia is and which fish suit your water. | At set-up, weekly while cycling, then monthly |
| KH (carbonate hardness) | Buffers pH. A very low KH lets pH swing suddenly, and the cycle uses it up. | At set-up, weekly while cycling (more often if it is low), then monthly |
| GH (general hardness) | Matters for shrimp, livebearers and many soft or hard water fish. | At set-up, and if your source changes |
| Temperature | Sets the pace of the bacteria and suits your species. | Check every day |
About pH
A stable pH matters more than hitting a textbook number. Look up the range your species live in, and avoid chasing it with pH-adjusting products, because repeated swings are harder on fish than a steady value slightly off target.
Tap water first
Always use a dechlorinator that handles chlorine and chloramine. It is worth testing your tap water for ammonia, nitrate, pH, KH and GH once at the start, so you know what comes out of the tap before it reaches the tank.
Common new-tank mistakes
- Adding too many fish at once. The bacteria cannot grow fast enough. Add a few at a time and test after each batch.
- Rinsing the filter under the tap. Chlorine kills the bacteria you spent weeks growing. Use old tank water.
- Overfeeding. Every uneaten flake becomes ammonia.
- Panicking at the first spike. Ammonia in a cycling tank is normal. Know the pattern so you can tell expected from alarming.
- Not keeping notes. A week of readings makes it obvious where you are in the cycle.
Log the cycle as it happens
TankSnap keeps each test in a timestamped history for your tank, so you can see ammonia peak and fall without hunting through scribbled notes. Photograph your strip or kit and confirm the suggested reading. Nothing is saved without your OK.