Wikipedia
El-Fish is a fish and fish-tank simulator and software toy developed by Russian game developer AnimaTek, with Maxis providing development advice. The game was published by Mindscape (v1.1) and later by Maxis (v1.1 + v1.2) in 1993 on 5 diskettes.
Each fish in El-Fish has a unique Roe, similar to the genome. This allows the user to catch fish and use selective breeding and mutation to create fish to their own tastes for placing them in virtual aquariums. Around 800 possible genetic attributes (fin shape, body color, size, etc.) are available, which can be selectively shaped into virtually infinite numbers of unique fish. Once fish are created, El-Fish will algorithmically generate up to 256 animation frames so that the fish will appear to swim smoothly around the tank.
The tank simulator is very customizable for this game's era. The player can select from a large number of backdrops and tank ornaments for the fish to swim between. The user can also import their own images to use as tank ornaments. El-Fish includes a fractal based plant generator for creating unique aquarium plants. There are several "moving objects" that can be added to the tanks which the fish will react to, such as a cat paw, fibcrab, and a small plastic scuba diver. The user can also procedurally generate background music for their tank using an in-game composer, or choose a separate MIDI music file to be played for each virtual aquarium, and feed the fish.
The tank simulator can run as a memory-resident program in MS-DOS, making it a screensaver. Multiple tanks can be displayed via El-Fish's slideshow feature.