Stopping migration?

Migration between empires can be avoided simply by not having any such agreements with another empire. Migration between colonies of your own people can be nullified by having all colonies with Optimal conditions (some climate control facility under Planet Utilization tech) and/or by having allowed all colonies to grow so they're full. Without anyplace for them to migrate to, they won't migrate. And if there's no place better to go to .. they might stop as well. Not sure on that one.

Why should you stop the migration? I think migration is one of the best benefits, because you can have different races with different breathable atmospheres. Many of your domed colonies can be used not-domed and so they can hold more facilities.

Best regards,
Pit Fiend

Yeah it just needs tweaked so that the entire population of a race can migrate off a planet if they don't like the atmosphere.(assuming this wouldn't leave the planet uninhabited)

Ok, if you rely on RPG style and don't want any other race on your planets, then migration really hurts. Migration depends on race style and so i think there's no option to stop migration entirely.

Best regards,
Pit Fiend

If those alien races are dumb enough to migrate to your world you can always just space them...
Of course this takes time but hey it's good for role play
Groovy Baby Yeah!

I usually played MoO3 without colonizing most of my planets. I had such a wide array of races in my empire that they'd start migrating to planets on their own and expand my empire without any effort on my part. Magnate civilizations RULE! In SE5, having multiples races will allow you to get full pop on multiple planet types. That is really nice.

Try this to stop migration (settings.txt):

Race Starting Percent Migration := 0
Race Migration Added 1 Percent Per Population Amount := 50000

Um... That's not gonna help much. you'd need to change the second number to around 5,000,000

No, populations are always indicated in millions!!
Unless you have a fully populated sphereworld that should stop migration.