Quite a common problem experienced by fish keeper is losing fish after they have recently been
introduced to the aquarium.
There are 2 likely causes:
1. If the aquarium is a new set-up, then fish are lost through a phenomenon known as ‘New Tank Syndrome’
- but is more accurately described as an immature filter problem.
Fish are constantly excreting an invisible toxin called ammonia. If the ammonia level is allowed to
build up in the aquarium, then fish will become stressed, leading eventually to health problems and
It is the role of an aquarium filter to break this waste down into less harmful nitrites and then
nitrates. This detoxifying process is achieved by beneficial bacteria that gradually colonise the
filter, breaking down the harmful ammonia and nitrites.
If fish are added before sufficient bacteria have colonised the filter then these fish will suffer
from poor water quality, leading to their stress and ill-health. The same problem will also occur
if fish in a new aquarium are over-fed.
The solution to losing fish to New Tank Syndrome is to stock gradually and feed sparingly.
Fish should be introduced a few at a time so that the bacteria are given time to colonise the filter.
As the filter matures over the next few weeks and months, then more fish can be added with confidence.
Through this running-in period, the water should be tested for ammonia and nitrite.
Only if a ‘zero’ reading for each test is obtained is it safe to proceed. If an ammonia or nitrite
reading is recorded then carry out a 30% water change to dilute the problem and only commence feeding
when the bacteria have reduced the ammonia and nitrite levels to zero.
The first two months of an aquarium’s life are its most critical, and with a few careful checks during
the running-in period, the aquarium can be matured very safely without the risk of losing newly-introduced fish.
2. Another cause of losing newly introduced fish is through acute stress
Choosing, netting and bagging fish at a shop are unavoidable practices when buying fish, but unfortunately
they are unnatural to the fish and cause them stress. The water quality that they are kept in while at the
shop will also be different from that of your home aquarium. Consequently, fish can also be stressed when
they are introduced to your home tank due to the change in water conditions.
How stress can be kept to a minimum
Once caught and bagged, fish should be transported in an opaque bag or container to shield them from
harsh or intense light. The bag should also be handled carefully and taken home as quickly as possible.
Upon introducing the fish to your home aquarium, the following steps should keep fish stress to a minimum.
1. Turn off the aquarium lights
2. Float the bag(s) for 5 minutes to equalise temperatures
3. Open the bag and roll back the neck, adding some aquarium water to the bag
4. Float for a further 3 minutes and introduce the fish by tipping out the bag
5. Turn the lights on and add a pinch of food to distract the existing fish from the new additions
6. Observe the other fish to see that they do not bully or nip the new fish