Been forgetting to post this bug everytime i see it happen.
Fish will lose the ability to pathfind until after reload if for whatever reason their liquid they are in changes somehow from the bottom up. This will always happen when another liquid falls in that displaces their liquid and you mop it out using a duplicant.
Have a pool of liquid with fish in it. If using polluted water use a denser liquid such as petroleum to sink to the bottom, or polluted water if using water for the main liquid for the fish. Have a duplicant mop the liquid out and notice that the fish become stuck in the spot above where the liquid was mopped from until reload.