I would probably want a picture. There are TWO major types of cucumbers - a filter feeder, and a sand sifter...and both need certain care. You need to be sure which one you are getting. The filter feeders, in particular, can be toxic, and they also need a lot of filter feeder food. Sand sifters need particular types of substrate.
I would start with some specifics about your tank - parameters, age, inhabitants, and we'll go from there.
These are animals that are not for young tanks, and they are very very sensitive to water parameters.