Just leave it in for a couple of hours - no more
With fry needing higher water quality than the adult fish, you want to cut down on polution as much as possible. I often leave cucumber etc. overnight for my large fish but will remove any leftovers by then next morning.
You might want to blanch the veggies to soften them up for your fry. Also, try growing algae in your fry tank. In nature, algae is a food source for fry.
You can stick a teaspoon or similar object through it to weigh it down. Blanching for a few seconds in boiling water (do not cook it!) will help for the fry as mentioned. I don't bother blanching for my adult fish.
Sometimes it takes a bit of getting used to. If they are too small, they might not be interested yet. If none of them have touched it in half an hour or so, remove it and try again in a few days
rofl, the fries are now under it while the adult guppies are eating it, i tried to look for the source but couldnt find it, it was in the forums of guppies.com
I found that my fish needed to get used to cucumber/zuchini first!! The first few times it went into the tank they didn't go near it, with a little perserverance though they soon became too inquisitive to ignore it!