Which fish simulator did George Fan first imagine Plants vs. Zombies as a sequel to?

George Fan first imagined Plants vs. Zombies as a sequel to Insaniquarium.

Insaniquarium was a 2001 PopCap game in which players maintained an aquarium, bought aquatic creatures, collected resources, and defended the tank from hostile aliens. Fan worked on Insaniquarium before creating Plants vs. Zombies, so his early concept reused the idea of an escalating defense game set in a domestic environment. In the prototype vision, aliens from Insaniquarium would invade a player's garden rather than fish occupying an aquarium.

The concept changed substantially during development. Fan decided that zombies would make the game more distinctive, especially as several gardening-themed casual games were appearing around the same period. The project also moved away from being a direct Insaniquarium sequel and developed its own plants-versus-undead identity.

The connection remains visible in the finished game. Zombiquarium, a mini-game featuring aquatic creatures and zombie-themed aquarium play, acts as an affectionate reference to Insaniquarium. That cameo often causes players to assume the two games share a formal story continuity, but the important link is developmental inspiration: Insaniquarium supplied the starting idea, not the final setting or plot.