Why new fish die soon after you buy them

Updated on August 26, 2026 · ~7 min read

In the great majority of cases the cause is not the fish but the tank: a nitrogen cycle that was never completed, acclimatisation done too fast, or too many animals added at once. Before buying again, measure ammonia, nitrite and nitrate: three zeros in a young tank mean the cycle has not started yet.

You bring a small group of fish home, and within a week two of them are dead. Then you go back to the shop, replace them, and those die too. It is the story that makes most beginners give up — and in nearly every case the cause is neither bad luck nor the quality of the fish.

The causes, in order of real frequency

1. The tank was not ready (the nitrogen cycle)

This is cause number one by a long way. A tank set up a few days or a few weeks ago does not yet have the bacteria that process waste: ammonia and nitrite build up and poison its inhabitants within days. It is called new tank syndrome, the symptoms are fish gasping, sitting still or rubbing themselves, and no treatment works other than diluting and waiting.

⚠️ The trap that catches everyone: in a brand-new tank ammonia, nitrite and nitrate are all at zero, so the test "says everything is fine". Three zeros in week one mean sterile water, not tank ready. The tank is ready when ammonia and nitrite are at zero and nitrate has appeared.

2. Acclimatisation done too fast

The fish arrives in a bag of shop water, which may differ from yours in temperature, pH, hardness and — in marine tanks — salinity. Tipping it into the tank is an osmotic and thermal shock that sometimes kills on the spot and more often over the following three days.

How to do it properly:

1. Tank lights off, to reduce stress.
2. Sealed bag floating in the tank for 15-20 minutes: this equalises the temperature.
3. Then open it and add tank water a little at a time, a glass every 5-10 minutes, for at least half an hour. For delicate animals (shrimp, marine invertebrates, corals) use a drip line for an hour or more.
4. Lift the fish out with a net and put it in the tank. The bag water does not go into the tank: it holds ammonia accumulated during transport and possibly pathogens.

⚠️ One counter-intuitive point: opening the bag raises the toxicity. During transport the accumulated CO₂ keeps the pH low, and at low pH ammonia sits in the nearly harmless NH₄⁺ form. On opening, the CO₂ escapes, pH rises and part of the ammonia becomes toxic NH₃. That is why acclimatisation should not be dragged out for hours: half an hour to an hour of dripping is the compromise, more is not better.

3. Too many animals at once

A freshly cycled tank supports the load its bacteria grew on, not three times that. Stocking it fully in one afternoon triggers a second ammonia spike in a tank that seemed fine — the least recognisable variant of the same syndrome.

The rule: a few at a time, with ten days or so between one addition and the next. And the question before buying is how many actually fit: see how many fish in a tank.

4. The wrong species for that tank

A fish that reaches 25 cm (10 in) in a 60-litre (16 gal) tank does not die on day one: it dies months later, and nobody connects the two. The same goes for a hard-water fish in very soft water, for a shoaling animal kept singly, or for a peaceful one housed with an aggressive one.

Before buying it is worth checking adult size, minimum volume, temperament and required parameters: see freshwater fish compatibility.

5. The fish was already sick

It happens, and it is the cause people suspect first when it is actually the last. Signs to look for in the shop, before buying: fish swimming badly or sitting in a corner, clamped fins, a sunken belly, white spots, patches, laboured breathing, and — above all — other dead fish in the same shop tank. A serious shop does not take offence if you ask to wait a few days.

6. Disease triggered by stress

Transport is a significant stress, and stress lowers the defences. White spot disease typically appears 3-7 days after introduction: not because the fish "caught it" in the tank, but because it was already carrying it and can no longer hold it in check. That is why a quarantine of 2-3 weeks in a separate tank, for those who can, also avoids infecting the inhabitants already there.

What to do now, if it is happening

1. Measure ammonia, nitrite and nitrate. This is not optional: without those three numbers you are guessing, and the fix is different in each case. Many shops will test a sample for free.
2. A 30-50% water change if ammonia or nitrite are above zero, repeated daily until they are back to zero.
3. Suspend feeding for 24-48 hours. No healthy adult fish suffers, and food is the main source of new ammonia.
4. Do not buy more fish. It is the worst instinct: adding animals to a tank that is killing them increases the load exactly where it is already too high.
5. Remove the dead immediately. A dead fish behind an ornament is a source of ammonia feeding the problem.
6. Check the temperature with a second thermometer, and that the filter really is running.

How not to end up here

• Cycle the tank first, with fishless cycling or with filter media from a healthy, established tank (the most effective shortcut there is: it does not add bacteria, it transplants them fully grown).
• Choose inhabitants to suit the water you have, rather than correcting the water for years.
• Acclimatise calmly, lights off, without pouring the bag water in.
• A few animals at a time.
• Quarantine, if you can.
• Feed less than feels right: whatever is not eaten in two minutes becomes ammonia.

How Gurglee does it

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