I have 3 cucumbers in my 130 gallon reef tank. I have a black one, a pink one, and a tigertail. They all seem to do a good job. The black and pink ones spend more time out in view, where as the tigertail always stays hidden under the rockwork.
A 2 year old sandbed in a 50 gallon should be able to keep one going.
You can always add more sand alittle at a time to build it up a little thicker.
Most of the tanks I have set up for myself and other people usually only have about 1 1/2 to 2 1/2 " of sand in them. They range from 20 to 175 gallons and they all support cucumbers no problem. I put 3 tigertails in the 175 originally and it has at least 6 in it now.
PS. You are going to hell for having a yellow tang in a 50 gallon tank.
