There's another answer to this other than getting your babies eaten.
Change the water chemistry. In a working tank, you have to do this graaaduuuallly....
... but: it works.
The two variables that I have played with are pH and Carbonate Hardness. I am pretty sure that your own experience will vary, but I find that very softwater (no Carbonate Hardness) and relatively low pH for Guppies (about 7) produces few if any babies.
At te other end of the spectrum, at 7.5-8pH and hard water (high Carbonate Hardness) my tank fills with babies almost as I watch (exaggeration, but it's dramatic).
Let me say again that you have to vary these parameters slooowly (certainly no more than 0.1 pH per day) otherwise fish will die from shock. But with care, you can use this to control breeding rates.
GaJ
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