If it's smaller than you, you can guide your fish towards it and eat it. If it's bigger than you? Well, you're going to get eaten yourself, so make sure you stay the hunter rather than the prey.
Every time you expand enough to take on a new opponent, you'll receive a pop-up announcement about what's next on the dinner menu.
Each tank stage will present its own unique challenge.
Keep an eye out for anything lurking in the shadows, as it's usually preparing itself to jump into the foreground and finish your game for you.
Simple and effective move: if there's a juicy fish waiting to be gobbled up, but a game-ending creature is in the way, you can often tease them into action by getting close, then running away. It will take a while for the creature to reset, leaving you free to grab your dinner.
Don't forget to grab any radioactive canisters they will give you some of the precious Zomboil currency, used for buying more powerful fish and tank upgrades.