Several factors here.
[/QUOTE]I recently bought an aquarium (125Litres) and filtered the water for the recommended time and heated it to 24 degrees. [QUOTE]
The tank has probably not established its nitrogen cycle and is poisoning the fish. What was the recommended time to run before adding fish? You were probably misled on when it would be safe to add fish.
You did not post ammonia or nitrite test results, did you test for these?
Unforturnately, the fish you chose are woefully incompatible despite the research you did.
The tank is also overstocked with fish.
Instead of killing the fish, return them all to the store.
Make sure you have a liquid drop test kit (not strips) that test for ammonia, nitrite, nitrate and ph.
Follow the instructions in this AA link.
While you wait for the tank to cycle that will have no fish in it, do more indepth research on the number of fish you can safely keep in a 125 liter and compatibility.
I'm sorry you got such rotten advice to begin with.
But its not too late to start over and do the right thing by your future fish
Once the tank is fully and completely cycled