In that case, I would just cut a round slice off the cucumber. Some people weigh it down by tying it to a rock with an elastic band. You might be able to push it into the gravel - never tried cos my plecs really go for it so it would soon float back up. YOu can try a plant weight or you can get veggie clips from your LFS. At the moment, cos I put in about a 4 inch slice, I stick a stainless steel dinner knife in it!
The shark will only nibble at it, yours is about the same size as mine - cute aren't they? My mollies and my ram seem to like it too but they won't eat nearly as much as a plec would. Just leave it in for a few hours and see how many eat it. When it starts to go fluffy or they have their fill, whip it out.
