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I used to have Platies and you can tell when they are pregnant because they have a belly. They are live bearers which means they give birth to their babies, so they don't lay eggs. Flankly I do not know what other fish look when they're about to lay eggs, but I have never read in any fish book that you can see their bellies get big before they lay eggs. To read more about live bearing fish, please look at this page:

"Species of interest to aquarists are almost always members of the family Poeciliidae, most commonly guppies, mollies, platies and swordtails."

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