"if ice froze and sunk, the next incremental bit of water on top of the ice would freeze and sink, so as you guessed the oceans and lakes would freeze over and no fish and no plankton"
If ice were denser than water then it would only sink a certain amount until it melted as pressure and temperature increased. So why would ice being denser than water make the layer of ice on the surface any thicker than it already is?
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