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 Oxigénio ) 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: Valores da água .
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.
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!