Teacher had a fish that floats in water. He floated in a 2litre bottle.
He had his own air tank (a biro cap, with no hole in the top, held on to the fish with a paper clip and elastic band). We had to figure out a way to get the fish to sink and then float again. 
We tried turning the bottle upside down (couldn't do this in the sea though), shaking the bottle and holding it sideways. 
The way to get the fish to sink was to squeeze the bottle. When you do this, the fish sinks to the bottom, and when you stop squeezing, the fish floats again.
This happens because the air in the pen top helps the fish float. When you squeeze the bottle, the air in the cap gets squashed a little (it compresses). This means that the bottle fills a little with water instead. Now the fish hasn't enough air to keep him floating, and the fish sinks.
When you stop squeezing, the air in the pen cap expands again, and now there is enough air in the cap again so the fish can float. We also tried this with a diver and it works in just the same way.
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