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Sink or swim

The OTAGO MUSEUM shows you how to create your own model fish and helps you find out how these clever creatures move so smoothly in the water.

Fish are remarkable animals; they can swim up and down through water with minimal effort. Have you ever tried swimming down to the bottom of a pool and discovered just how hard it is to stay down there? Many fish have a unique organ that helps them float and sink – it’s called a swim bladder, and it’s filled with air. To get an idea of how this works, we can make our fish, with its very own swim bladder. 


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  • A plastic bottle
  • A small balloon (such as a water balloon) 
  • Blu Tack 
  • Water


  1. Blow up the balloon with air, but not too much – you want it to be able to fit into the top of the bottle. 
  2. Then you want to create a fish tail. For this, use Blu Tack, this will add some weight to your fish. You should try to use as much Blu Tack as you can, without your fish sinking – stick this around the tied-off end of the balloon.
  3. Fill your plastic bottle to the top with water. Put your fish into the bottle, tail first, and screw the lid on tight. 
  4. Now you can see if your fish knows how to swim up and down. Firmly squeeze the plastic bottle, and your fish should start to move down. When you release the pressure, your fish should swim back up. 

When you squeeze the bottle, you’re also squeezing the balloon. As the balloon gets smaller, your fish goes down. When you let go, the balloon gets bigger, and your fish floats back up. Fish can use their swim bladders to make it easy for them to move up and down in the water.

In Otago Museum’s Tūhura Tropical Forest you can watch our fish do this. It’s as easy for them to swim as it is for us to walk. 


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