Depending on your breeder size I think , in my case , I have two breeder , one is a much larger one like a big square net where the other one is a plastic made small one , I don't cover the top for both but turns out usually the guppies in the small one are expressing much more stress and they will try to jump back into the main tank area .
I won't say it's unsafe to put them in a small breeder but seems like they are more vulnerable in a small one . Generally speaking , I heard ppl usually say it's best not too keep them for too long ( few days ) but I don't think there is a golden rule for that , I got friend who put the guppies for two weeks and are just fine , the idea is they seems getting more stress when they realize their got locked up , if we want to minimize the risk that they got too much stress in it ,it's always better to keep it as short as possible.
On the other hand , some ppl believe it's better to put them into a breeder early (like one week before giving birth )
However instead of timing it , personally I think the best way is to have a reasonable size of breeder , and I prefer those which we put right into the tank instead of an external one , I found this give me the best result
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