The general rule of thumb is to allow one inch of fish per gallon of water. This means that a 24 liter tank could only comfortably hold two 9cm fish.
Putting two 18cm fish in a 24 liter tank would overcrowd the tank and could lead to health problems for the fish, including:
* Stress: Overcrowding can cause fish to become stressed, which can lead to a weakened immune system and make them more susceptible to disease.
* Competition for resources: Two 18cm fish in a 24 liter tank would have to compete for food, space, and oxygen, which could lead to aggression and injury.
* Poor water quality: Overcrowding can also lead to poor water quality, which can further stress the fish and make them more susceptible to disease.
For these reasons, it is not advisable to put two 18cm fish in a 24 liter tank. If you want to keep these fish, you will need to upgrade to a larger tank.