The item pictured above is a standard guppy breeding trap available at places like Petco. Most of the time, these aren’t a great idea. But over the years, it’s helped solve problems. Way back when, I used to have guppies that really didn’t eat many of their fry. So a couple clusters of fake plants floating at the top was enough to give them hiding. As I added new types of guppies, I frequently noticed that there appeared to be pregnant females, but they only gave birth to a few fry here and there. What I didn’t realize was that as quickly as those fry were being born, they were being gobbled up in the tank. And a few stray ones would go unnoticed long enough by the parents to be noticed by me.
As my “program” grew over the years, I changed my approach. The guppy master breeders had this one right. When you have a pregnant female who appears about ready to give birth, you pull her out of the group and put her in a tank by herself. Then she gives birth to 20-60 fry, then you put her back in the main tank with the other adults. Most of the time, this works very well. Then you don’t have to net out 60 babies from one tank and move to another tank. You net out one slow big easy-to-catch fish and you’re done.
However, some guppy strains are particularly aggressive at eating their young. I heard Stan Shubel mention that whenever he sees a female guppy eating her young, he “disposes” of her. And supposedly this has led to strains of guppies that don’t eat their young. Though I’m not sure on the logic of this, because the mother eating her young just gave birth to 40 fry. And they all have the same DNA as the cannibal mother. In theory, you’d have to throw out the mother and the fry if you wanted to end that characteristic.
If I see one of my more aggressive female guppies starting to give birth, I’ll move her into one of these traps. I’ll float this trap in an empty tank (empty of other fish, not water), then place her inside the trap. She starts having fry, which fall through the floor of the trap. And from there, they can swim out the slits on the side. Once she’s stopped having fry, I can put her back in the main tank. This keeps mother and fry apart from start to finish (mostly). The only time I ever use these traps are when the mother is already giving birth to fry. They don’t work well if you put a fish in here and float it in a community tank. The fry would just escape into the community of other fish where they can be eaten. Often if you leave a fish overnight in one of these traps, they’re likely to jump out and land in the same water with the fry. So I recommend keeping your eye on the trap, and when she appears to be done giving birth, move her back to the main tank.