Feed your fish. Select the Food tool (bottom of the screen, or hotkey 1) and click in the water to drop a pellet. Fish swim over to eat it.

Watch the hunger icons. A "!" means a fish is getting hungry and hunting for food; "!!" means it's getting desperate — leave it too long and it starves.

Buildings can go anywhere in the city. Nothing has to touch anything else down there. Platforms are just a cheap, optional flat surface for catching items before a Fan or Collector grabs them.

Fans blow coins, Food, and Waste back up into the water column so your fish (and you) can reach them, instead of everything piling up out of sight.

Collectors automatically pull in and bank any coin or Science Bubble touching them, from any side — no clicking required.

Refineries absorb Waste and dispense fresh Food (or Bio-Sludge into Biomass) — Electric, Advanced, and Bio tiers process faster.

Click a coin to bank it yourself any time — Collectors are a convenience, not a requirement. Waste can be left for a Suckerfish or a Refinery to clean up.

The Mound (seabed, early game) cracks open for gold, unlocking new buildings and fish as you feed it money. It eventually shatters into...

...the Science Lab — the long-term goal. Buy Suckerfish and Science Octopus with gold alone to get started, then spend Science Bubbles (dropped by the Octopus) and gold on the rest of its branching tree to unlock Advanced/Bio buildings and every Gene-Spliced hybrid fish.

The Merge tool (hotkey 3) lets you drag one Adult fish onto a matching one to combine them into a bigger, shinier fish, or drag an Adult Suckerfish/Eel/Octopus onto another Adult fish to splice a hybrid.

Tank Points (earned every time a fish grows to an adult) buy permanent upgrades in the Tank Upgrades panel — faster fish, better food, bigger coin caps, and more.