Install from terminal :
~$ sudo apt-get install fillets-ng
or download from http://fillets.sourceforge.net/download.php
How to play ?
After running the level you can save (F2) or load (F3) the game and abort (Escape) or restart (Backspace) the current level.
When saving the solution you must keep in mind that there is only one slot for load and save for each level. When saving new position the older one is erased without asking for confirmation. It is recommended that you save new position only when you are sure that you are going in the right direction.
Other useful actions you can do are to show help (F1), show move counter (F5), show/hide subtitles (F6), increase game speed (hold down Shift), show game menu (F10) or switch to fullscreen (F11).
Different options are in game menu which can be invoked by clicking Options in the right bottom corner in the world map or by F10 key. There you can control the volume of music and sound, you can choose the language of subtitles or turn subtitles off completely.
You can control your fish in one of three ways.
1. Arrow keys
Up, down, left and right arrow keys control the current fish. You can use space bar to switch between the fish.
When the fish doesn't move although you are pressing the keys you are probably trying to push something that cannot be moved or you are trying to push the steel object with the smaller fish.
If you press and hold the arrow key, the fish movement will accelerate after approx. three spaces.
Point the mouse cursor at the place where you want your fish to go, press and hold the left mouse button. If the fish can get to this place, it starts moving. Its movement will accelerate after approx. three spaces.
If you want to move something, press and hold the right mouse button. The currently selected fish will try to move directly to the spot where you clicked, pushing aside everything in its way.
You can switch between fish simply by left-clicking the one you want to activate.
3. Direct control
Keys A, S, D and W control the smaller fish and keys J, K, L and I control the bigger fish. The only advantage of this control is that you need not to switch between the fish.
for more detail to play visit here.