Fish Fillets NG is a puzzle game. The goal in each level is to find a safe way out for both of the two fish. It's comparable to other sliding puzzle games such as sokoban and klotski. In Fish Fillets the play is complicated by a few additional elements and rules. The game has background music and, on many levels, some animated effects, though neither normally has an effect on game play.
The game was originally commercial and was first developed by Altar Games and released in 1998. In 2002, it was relicensed and released under the GPL. In 2004 fans created a new version ("Next Generation") based on the open source and ported it to many other operating systems and languages.
The game involves two fish both controlled by the player moving various objects and obstacles around until they may both safely exit the level. Unlike many other sliding puzzle games, this one has gravity and an unsupported object will fall until it lands on something else. If an object falls onto one of the fish, it dies and the level cannot be completed unless restarted. One of the fish is larger and only he may lift or push certain objects (steel objects), which adds another level of complication to the puzzles. Another important, complicating rule is that no object may be slid across the back of a fish, as this is considered to be the same as an object landing on a fish's back. However, a fish who is holding up an object may swim back and forth underneath it and an object may be slid exactly one unit off of a fish's back if it is moving onto solid support in the process.