[rininger85]

as long as the breeder box is in the same tank and isn't filling up with waste she'll be fine for as long as you need to have her in there. Guppies are very hardy, the only issue I see with breeder boxes is it could add stress, I've seen some attempts on here where people scraped up their breeder boxes so they couldn't see through to see the other fish and stress out that they are in the breeder box, and added some floating plants to the breeder box to calm them too, all seems to be a good idea to me...

 

[OAli]

I float plant trimmings in there till they root, keeps momma calm then helps the babies feel secure afterwards

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[Edub]

I've kept my guppies in breeder boxes for several weeks at a time. As long as they get good water flow and the box stays clean it shouldn't be a problem.

 

[JustJen]

I always tried to limit the time I kept mine in there, but it was usually several days - I certainly wouldn't be worrying over 12 hours.

As others have noted, a lot of floating plants will help reduce stress levels and also give the babies a place to hide. Keeping the babies safe from other tankmates isn't enough, momma is just as likely to gobble them up as anyone. And in my experience with a lot of guppies over the years, every albino strain I've ever had were BRUTAL baby eaters. I don't know if that's because the babies are easier to see or something specific to albinos or what, but they were downright relentless about eating those babies!