Answer: The required equation :

There are 13 fishes in small aquarium.
Explanation:
Let the number of fishes in small aquarium be x.
Given : A large aquarium has 6 more fish then a small aquarium.
There are 19 fishes in a large aquarium.
Then, the equation to find the number of fishes in small aquarium (x) is given by:_

To solve the above equation, subtract 6 from each sides , we get

Hence, there are 13 fishes in small aquarium.