Shameless re-post from the 3.04 thread,  which I will continue to re-post every patch until I get a dev response of "thanks we'll think about it" or "thanks but no thanks,  we like it the way it is".
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Can you please put in some code that destroys all (or all but 1) existing pop when a planet is conquered by an incompatible civ?
I would define "compatible" as Carbon/Aquatic with Carbon/Aquatic.   Synthetic only with Synthetic,  and Silica only with Silica.
It is slightly rediculous,  immersion breaking,  and game-breaking that a race that spends so much time and resource to build more pop gets unlimited instant free pop conversion if they take over a meat planet.    It is only slightly less rediculous (but more amusing) that when a meat race takes over a synthetic planet,  the robots all start having babies.
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(I almost want to say, "for £5 I will not interrupt this patch!")