I have found that you need to experiment with your specific fish. I have tried to feed my livebearers cucumber and zuchini, prepared various ways, but to no avail. Others on this forum have said that their fish love these vegtables. My mollies seem to like spinach though.
If they don't eat whatever you give them make sure you remove it from the tank within 5 minutes, so that it does not start to degrate. Also remember to wash the vegtables very well as they will often contain pesticides and dies. Organic grown produce is best if you want to spend the money.
Good luck