El-Fish is presented as an aquarium-management and breeding simulation. Players create or acquire fish, place them in tanks, and observe how their physical traits appear and develop. The main objective is open-ended: build collections, experiment with inheritance, and produce unusual or attractive fish rather than complete a fixed campaign.

The game is organized around aquarium and breeding activities rather than discrete action stages. Players can select compatible fish, manage tank conditions, arrange displays, and save successful specimens. Progress comes from learning which traits pass to offspring and refining generations through repeated breeding.

Begin by breeding fish with clearly different traits so that inherited changes are easier to identify. Keep successful parents and offspring recorded separately, because a visually attractive fish may also carry useful traits that are not immediately visible.

Maintain suitable tank conditions and avoid overcrowding. Use controlled pairings instead of mixing every fish together, then compare offspring across several generations. Save distinctive specimens before making further experiments, since the value of a fish may depend on a rare combination of traits.