Try peas. I've tried a few different veg with my various fish and not even the bristlenoses touch the cucumber. *doh* I think maybe mine all have far too much so simply aren't interested but peas often disappear where other things don't. And they don't break apart as badly as cucumber left in overnight does. Blech.
You can also try frozen or freeze dried foods for your fish if you want to avoid feeding alot of flake. I also fine fresh (cleaned and pealed) king prawn is very popular with alot of fish, especially bettas.
Alright, am blathering. I'll shut up now.
Hugs,
P.