These are not pets. They are specimens.

In Tank of the Fittest, you take control of an industrial aquarium on the edge of collapse. Twist heat, pH, oxygen, and pollution to create deliberate survival pressure, then watch ordinary fish become casualties, predators, or evolving anomalies.

Turn the tank into a selection engine

The environment is not a comfort checklist. It is your primary tool. Raise the heat, shift the pH, starve the water of oxygen, or let pollution spread to drive different specimens toward different outcomes.

Make death part of the experiment

The weak leave behind food and biological material. Their remains attract scavengers, feed predators, alter pollution, and keep the ecosystem moving. One death can strengthen a hunter or tip the entire tank into a new collapse.

Build around the survivors

Specimens gain traits, form synergies, and follow distinct evolution paths in response to clear, predictable pressures. Commit to a few focused build paths and turn one survivor into an apex specimen dangerous enough to dominate the entire tank.

Archive what the tank creates

Advance the experiment through tasks, unlock new fish lineages in the Mutation Codex, and record the specimens and traits you discover. Archive the results of each run and turn unfinished discoveries into goals for the next experiment.

• Control heat, pH, oxygen, and pollution
  
• Watch pursuit, escape, devouring, scavenging, and evolution form ecosystem chains
  
• Build key specimens around trait families and synergies
  
• Unlock 15 base fish lineages and record 45 specimen forms
  
• Explore 117 obtainable traits and 36 synergies