Answer:
12:7:21
Explanation:
If you have 40 total, the non-bram are (40-12) = 28. One quarter of them, or 7 (since 7x4=28)are trench. It leaves 28-7= 21 carps. The total ratio is: 12:7:21. Since the three numbers are co-primes (ie, they don't share a divisor between all 3) there is no way to simplify the ratio (or, if you prefer, their sum, 40, is the minimum ammount of fishes you need to have a whole number of fishes in the pond).