there are some angelfish and 18 swordtails in an aquarium. The ratio of the number of angelfish to the number of swordtails in the aquarium is 1:3. How many more swordtails than angelfish are there in the aquarium.
The ratio of angelfish to swordtails in the aquarium is 1:3, which means that for every one angelfish, there are three swordtails.
Let's say there are x angelfish in the aquarium.
Therefore, the number of swordtails in the aquarium is 3x, as per the given ratio.
The total number of fish in the aquarium is x + 3x = 4x. (Combining the number of angelfish and swordtails)
We are given that there are 18 swordtails in the aquarium, so we can equate 3x = 18 and solve for x.
3x = 18
Divide both sides by 3: x = 6
So, there are 6 angelfish in the aquarium.
And there are 3 * 6 = <<3*6=18>>18 swordtails in the aquarium.
The difference between the number of swordtails and angelfish is 18 - 6 = <<18-6=12>>12. Answer: \boxed{12}.