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AquaReefBasin

Cycle and parameters

My new tank is reading ammonia and there are already fish in it

A tank under about eight weeks old with an ammonia reading is not broken — it is unfinished. The bacteria that consume ammonia have not grown to the size of the waste stream yet, and until they do the only thing that removes ammonia is changing water. Test today, change 25 to 50% depending on the reading, and stop feeding until it comes down.

Act today

What this problem usually shows in the water

Test these first

Before anything goes into the tank and before anything comes out of it, get these numbers. They cost fifteen minutes and they decide everything that follows.

  1. Ammonia — total ammonia — the ammonium ion plus free ammonia, added together
  2. Nitrite — nitrite ion, the intermediate product between ammonia and nitrate
  3. pH and KH — acidity of the water, plus the carbonate buffer that resists changing it

What is usually behind it

Causes at a glance
CauseHow to confirmTime to resolve
The tank was stocked before it was cycledThe tank is under two months old, fish went in within days of setup, and both ammonia and nitrite are moving. This is the ordinary case, not a rare one.Four to eight weeks to a finished cycle
Too many fish added at onceThe tank was reading zero and then started reading ammonia within a week of new arrivals. The colony was sized to the old stocking level.One to two weeks for the colony to catch up
Chloramine in the tap water reading as ammoniaTest your tap water directly. If it shows ammonia, your utility uses chloramine and your conditioner is splitting it — the ammonia half is genuinely there and genuinely being read.Twelve to twenty-four hours per water change, once the filter is running
The filter media was replaced or rinsed under the tapThe reading started one to three days after filter maintenance. The cartridge was swapped for a new one, or the sponge was held under running water.One to three weeks to regrow what was lost
Something has died in the tankCount the fish. Look behind the filter, inside decor, and under the substrate edge. A smell alongside the ammonia reading makes this much more likely.One to two days after removal

The tank was stocked before it was cycled

How to confirmThe tank is under two months old, fish went in within days of setup, and both ammonia and nitrite are moving. This is the ordinary case, not a rare one.

What to doChange water daily or every other day, sized to the reading: 25% at 0.25 ppm, closer to 50% above 1 ppm. Feed lightly every second day. Test both ammonia and nitrite every day and keep a written record — the numbers tell you where you are on the cycle.

Time to resolve: Four to eight weeks to a finished cycle

Too many fish added at once

How to confirmThe tank was reading zero and then started reading ammonia within a week of new arrivals. The colony was sized to the old stocking level.

What to doChange 30% now, cut feeding in half, and add nothing further living until the reading holds at zero for several consecutive days. The colony grows to match the load, but it takes one to two weeks to do it.

Time to resolve: One to two weeks for the colony to catch up

Chloramine in the tap water reading as ammonia

How to confirmTest your tap water directly. If it shows ammonia, your utility uses chloramine and your conditioner is splitting it — the ammonia half is genuinely there and genuinely being read.

What to doKeep treating every drop of replacement water by the label. In an established tank the filter processes this small addition within a day. In an uncycled tank it adds to the load, which is one more reason to change water in moderate amounts rather than enormous ones.

Time to resolve: Twelve to twenty-four hours per water change, once the filter is running

The filter media was replaced or rinsed under the tap

How to confirmThe reading started one to three days after filter maintenance. The cartridge was swapped for a new one, or the sponge was held under running water.

What to doPut the old media back in alongside the new one if you still have it, even if it looks filthy. Change water on the readings from here. In future, rinse media in a bucket of used tank water and never replace it all in one session.

Time to resolve: One to three weeks to regrow what was lost

Something has died in the tank

How to confirmCount the fish. Look behind the filter, inside decor, and under the substrate edge. A smell alongside the ammonia reading makes this much more likely.

What to doRemove it, change 30 to 50% of the water, and vacuum the area it was in. The reading falls quickly once the source is out.

Time to resolve: One to two days after removal

What not to do

These are the moves that make this particular problem worse, and they are all common. Most of them come from wanting to do something substantial when the right answer is something small.

  • Do not change all the water at once in a panic. Large abrupt changes swing temperature and chemistry, and the fish are already under load.
  • Do not add a chemical that binds ammonia and then stop testing. Most of those products convert ammonia to a less harmful form and the test still counts it — the number staying up does not mean the product failed, and it does not mean you can stop changing water.
  • Do not clean the filter because things are going wrong. That is the opposite of what the tank needs this month.
  • Do not add more fish because the shop suggested the tank looks empty.

When this stops being a water question

Ammonia damages gills, so gasping, redness around the gills, sitting on the bottom or refusing food are all things you may see here. Change water immediately — that is the fastest thing that helps. If an animal remains unwell after the water reads clean, it needs an aquatic veterinarian; this site does not diagnose or treat fish.

Keeping it from coming back

The whole of this problem is a sequencing problem, and the sequence is: cycle the tank, then buy the fish. A fishless cycle takes four to eight weeks of doing almost nothing, and it ends with a tank that never puts an animal through this. If that ship has sailed, the next best prevention is to stock in small groups with two weeks between them, and to treat filter media as living equipment rather than as a consumable.

Reference for this page

Equipment involved

Related problems

Published sources

Published figures on this page come from the documents above. Where a value depends on species, pH or temperature, that is stated rather than averaged into a single number.