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What are the causes and what can be done about it?

It’s rare, but it can happen that several fish suddenly die and you have no idea why (please remove dead fish immediately). Since the reasons can really vary, we have to proceed systematically. A bit of detective work is called for now.

Check the standard elements are okay.

1. Is the filter running and does it run 24 hours a day?

The filter must NOT be switched off overnight! Then the beneficial filter bacteria living in the filter would die due to lack of oxygen and rotting processes would occur in the filter. When the filter is switched on again in the morning, all the rotting sludge would then be flushed into your aquarium.

2. Heating OK?

Check the water temperature. Heaters can also be defective and heat too little or too much. If the heater is defective and the water is too warm, you can immediately carry out a 30 % partial water change with cold tap water to lower the overall water temperature. Please do not forget the water conditioner JBL Biotopol in the rush and add it to the water quantity to be changed. If the water is too cold, check the temperature set on the rod heater and check whether the control lamp on the heater is lit. If it is on, the heater must be noticeably heated at the lower end. If it does not get warm, it is defective and must be replaced.

3. How are the surviving fish behaving?

If the fish haven’t all died, observe the behaviour of the remaining fish:

  • Are all the fish all swimming around normally in the aquarium or are they showing clear behavioural abnormalities? If the surviving fish are not showing any conspicuous behaviour, it’s more complicated. There may have been a problem that no longer exists. For example, in the case of a nocturnal oxygen deficiency, the weakest fish or the species most in need of oxygen would always die first. Poisoning/oxygen deficiency: Are the remaining fish floating near the water surface and gasping for air? Here the cause will be a lack of oxygen or poisoning.

Check the O2 content of the water ( JBL PROAQUATEST O2 Tlen ) with an oxygen test. The oxygen content needs to be above 6 mg/l. Measure in the morning, as this is when the oxygen content is at its lowest. If you take a water sample to a pet shop for an oxygen measurement, the transport container, e.g. a bottle, MUST be completely filled with water and no longer contain any air bubbles. Otherwise the O2 measurement will be falsified!

More information about oxygen: Parametry wody .

However, it can also be a symptom of poisoning. In cases of poisoning, the animals float near the water surface, lie on the ground breathing heavily or dart backwards and forwards in the tank.

There are 3 problem substances in water that cannot be detected with water tests: nicotine, pesticides (pest control agents) and herbicides (weed killers). Smokers should not smoke in the room where the aquarium is located! Nicotine is water-soluble and leads to symptoms of poisoning.

Pesticides and herbicides

Pesticides and herbicides can enter our aquarium (or pond) with the tap or well water. They come from agriculture and can be quickly and easily removed from the water by filtering through activated carbon (JBL Carbomec ultra).

Toxic nitrogen compounds

Toxic nitrogen compounds can be easily detected with water tests: Ammonia and nitrite could be a possible cause here. Phosphates and nitrates are not toxic to fish. Please do not test only nitrite! It can happen that the nitrite is not measurable, but the ammonium content is too high. At pH values above 7.0, part of the non-toxic ammonium is then converted to toxic ammonia in the water.

Details on these troublesome substances can be found here: Obniżanie problematycznych parametrów wody .

pH fluctuations

One problem that is not immediately obvious could be a pH fluctuation in the water. The pH rises during the day and drops overnight because of the CO2 content in the water. This is a completely natural process. However, the lower the carbonate hardness (KH) of your aquarium water, the more the pH fluctuates. If the KH is below 4 °dKH, the pH value can be, say, 6 in the early morning and 9 late in the evening. This would mean a thousandfold increase in the base content (opposite of acidity) during the course of the day, as the pH value is a logarithmic value (change by 1 = tenfold increase). If the pH value then drops again to 6 at night, there would be 1000 x more acids. If you only test the pH at noon, it would be around 7 and you would not suspect the fluctuation. Therefore, test the pH value once early in the morning and once late in the evening ( JBL PROAQUATEST pH 3.0-10.0 ) and also check whether the KH is above 4 °dKH ( JBL PROAQUATEST KH Twardość węglanowa ). By adding JBL Aquadur you can adjust the KH to the correct value and thus stabilise the pH value.

CO2 poisoning

There could be CO2 poisoning: If you have connected a CO2 fertiliser system for your plants to the aquarium, too much CO2 could have been added and this could have lead to fish death. The amount of CO2 in the water must not exceed approx. 30 mg/l.

Check the solenoid valve of your CO2 system, if you have one fitted. If it is not receiving any electricity, it is switched off. Only when power is supplied does it switch on the CO2 supply. This switching on can be heard by a click.

Perhaps you have added too much CO2? With the help of a CO2 water test ( JBL PROAQUATEST CO2 Direct ) you can quickly check whether the CO2 content is in the right range. The alternative way to check the CO2 content is a CO2 permanent test ( JBL PROAQUATEST CO2-pH Permanent ), which permanently shows you the CO2 content of your aquarium water. But be careful: this test does not react immediately to a change in the CO2 content, there is a delay of about one hour! If there is too much CO2 in the water, stop the CO2 supply immediately and aerate the water as much as possible. Stirring the water vigorously also expels CO2 and helps to reduce the CO2 content immediately.

Metal poisoning

Metal objects or metal pipes do not belong in the aquarium! Metals dissolve in the water and have a toxic effect on invertebrates and fish. Where can metals come from? Tap water is often fed to the tap through metal pipes. Stainless steel pipes are largely neutral, but all other metal pipes can release metal ions into the water and cause poisoning in our fish.

We humans also react negatively to higher copper levels in tap water. The newer the pipes, the higher the likelihood of metal poisoning. Replacing them with stainless steel or plastic pipes is therefore definitely a sensible investment, if at all possible. If you collect rainwater in metal containers or feed it through copper gutters to obtain soft water for your aquarium, these would also be extremely problematic quantities of heavy metals that enter your aquarium.

Unfortunately, tests can only detect iron (Fe), magnesium (Mg) and copper (Cu). We cannot test for other metals such as zinc, lead, tin or nickel. Fortunately, a good water conditioner like JBL Biotopol binds ALL types of heavy metals quickly and reliably. A battery dropped into the water can cause extreme poisoning! If you suspect heavy metal poisoning, take double the dose of the water conditioner to the entire water volume of your aquarium (length in cm x width in cm x height in cm : 1000 = number of litres).

Broken aquarium thermometers, unlike clinical thermometers, do not contain mercury and do not lead to poisoning. All JBL thermometers contain a non-problematic liquid (non-toxic alcohol and in very small quantity) and iron balls, so no lead or mercury! Collecting the broken parts and a 30 % partial water change with a double dose of water conditioner (JBL Biotopol) will prevent problems, because not everyone uses JBL thermometers.


It is highly unlikely that part of your fish stock will die from an illness overnight without showing signs of disease beforehand. When disease is present the weakest fish are usually the first to be affected and so not all the fish will die at once.

If ALL the fish have died, you will of course no longer be able to observe the remaining fish and you will have no choice but to check all the factors we’ve mentioned. If the aquarium is completely new, keep a close eye on the first fish you put in. If they show signs of poisoning again, the problem has NOT been solved by the new setup. Then you have to systematically go through every element in the aquarium. For example, we once discovered that a flexible aquarium background was releasing toxins into the water, leading to 100 % losses!

4. Fish dying directly after a water change

Check whether the tap water you added has lowered the water temperature in the aquarium too much. Some fish species react sensitively to sudden temperature drops. Remedy: Immediately drain some water again and refill with warmer water. Do not forget water conditioner JBL Biotopol !

Ammonia poisoning

The following situation may occur: You have an increased ammonium content (NH4) in your aquarium water, but this is not a problem as ammonium is non-toxic and the pH value of your aquarium water is just under 7. When you change the water, however, the pH value in the aquarium increases because your tap water has a pH value above 7. Now some of the non-toxic ammonium (NH4) becomes toxic ammonia (NH3) and the result is serious ammonia poisoning!

Details on ammonium/ammonia can be found here: Parametry wody .

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