Hello.
Remove any cc/mods that you have and see if that helps.
Try repairing the game again.
Try resetting your sims 4 folder and see if that helps. To do that, move your sims 4 folder ( that is in Documents -Electronic Arts - then the sims 4 folder is there ) to the desktop. Then start the game, and a new sims 4 folder will be generated. Start a new game and test to see if the fish remain in the tank. Exit the game and then add back the "options.ini" file and the "saves" , "screenshots" "tray" "custom music " and "mods" folders from the desktop sims 4 folder to the new sims 4 folder, replacing the new generated file/ folders with the old file/folders, testing after replacing each one to see how the game is working.