I don't get it. Am I supposed to play guessing game what numbers are going to move every fish at any given state or am missing something?
other than that this seemed like an interesting puzzle.
Ah, no, it's simply that the numbers are barriers. The fish will always try to go in whatever direction they are facing, but if there is a number blocking the way, they can't. If you do the math, you'll know what number you'll get, the object being to clear a path for the good fish without having them encounter a bad fish.
So like in the first level:
Right here, the fish is blocked because the top of the '0' is in the way, but after adding the '1' weight:
...now the fish can move because the number is not in the way anymore.