interestingly the skin hold most of the really good stuff. much of the required fibre is in that part. and if you par boil the veg, its even more important as the veg gives much of it nutrients and vitamins up, even with a quick boil, so its likely to be the only place with any real value left! it might be better, perhaps, to leave the cucumber/courgette on the shelf for a little while to let it soften up before you pop it in. don't worry, unless your filtration is totally up the spout, it will cause no problems. even if you just left it to rot in there. i use frozen peas, they just pop out of their skins, with a little push. this again avoids the need to boil and loose valuable nutrients. also, providing the salt content is checked, tinned veg of almost any type goes down well too.
ps, there are many fresh fruits that you can use too, though this depends on the fish. but they go down well with many plecos.