There is no single “Restart Game” button. You have three options depending on your goal:
Go to the main menu > Change Player > New Player. Enter a new name. This creates a fresh save slot with a new tank, starting funds, and no progress.
To completely erase your current profile:
%USERPROFILE%\Documents\Fish Tycoon\ or %APPDATA%\FishTycoon\~/Library/Application Support/Fish Tycoon/.sav or playerX.dat file corresponding to your profile.Warning: This is irreversible. Backup the file first if unsure.
In-game: Menu > New Tank. This starts a fresh aquarium under your current player profile, retaining your unlocked species/items but resetting the tank population and cash.