I would find the common factors between all of them, and settle with the factor 3.
If I have 3 tanks:
12/3 = 4
24/3 = 8
30/3 = 10
In each of the 3 tanks, there will be 4 angel fish, 8 sword fish, and 10 guppies.
I hope I helped :)
(I hope I didn't misunderstand the question, so if I did, I'm really sorry about that.)
