ye feed your pleco cucumber i fed mine about 2 hours ago he sucks out the inside and then all thats left is the dark green outline.
you can either stuff the cucumber under your gravel or decor or do what i do boil it for about 5 mins and then it will just sink to the bottom.
make sure its cool before you drop it in though
Remember to make the lettuce romain or spinach as its much more nutritious than ice berg lettuce as it's mostly just contains water anyway. My plecos all love cucumber more then lettuce though.
I slice the cucumber length ways, in sections of about 3-4 inches (I have big plecs) and skewer it on an ordinary dinner knife so it sinks and stays sunk.
Big plecs are not very delicate creatures and they need something steady to hold their food