Final answer:
The total mass of the aquarium when it is filled with water is 23 kg.
Step-by-step explanation:
To calculate the total mass of the aquarium when it is filled with water, we need to determine the mass of the water that fills the aquarium. The volume of the aquarium can be calculated by multiplying its length, width, and height (40 cm x 25 cm x 18 cm = 18,000 cm³). Since water has a density of 1 kg per 1 L of volume, the mass of the water is equal to its volume (18,000 cm³) in liters, which is 18 kg. Therefore, the total mass of the aquarium when it is filled with water is the sum of the mass of the empty aquarium (5 kg) and the mass of the water (18 kg), which is 23 kg.