My method: I take a small basin. 20x20x30cm. On the ground I place shards of a clay flower pot with the cavity down. In the middle of the water floats a Ceratopteris with long roots.
I can see if the female will give birth in a few days. She comes into this basin.
When she's born, the little fish hide under the shards.
In the cover of the plant roots they rise later secretly and lie on the leaf. I then take the female out. No young fish are lost.
I write the date in the log file. About 25 days, then it is again.
It is different. Guppies, Platies Mollies Swordfish. But around the 25 days.