In the question, it is not mentioned that which fish is dominant and recessive, so lets consider red fish is dominant and yellow fish is recessive.
Answer:
In complete dominance, the dominant trait completely masks the recessive trait and the offspring shows dominant characters only.
When a red and yellow fish were crossed, the pattern of inheritance is complete dominance which means the offspring would have dominant characters.
As we considered, red fish has dominant trait so the offspring produced will also have red color as yellow color (recessive) will be masked by dominant red color fish.