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.