My sand was dirty until I got a sea cucumber. A lot of people will warn against the cucumbers, but only because they are repeating what they hear and they don't have full knowledge of all the species.
There are two kinds of sea cucumbers. The first group are the brightly-colored filter feeders. These are the highly poisonous ones prone to fouling the tank when upset or killed. It is my opinion that these should probably be avoided.
The second group is the detrivore sea cucumbers. They tend to have drab colors to blend into their surroundings. They spend most of the day hiding and come out at night to feed. These guys use specialized tube feet under their "heads" to scoop up sand and detritus. After processing and digesting this, they expell it out the other end of their body. The reason I recommend these guys is because the sand they poop out is clean as a whistle!
I have gone as far as prying my cucumber off of a rock without having him get upset (a very stupid move, but included here just to make a point). It is my opinion that a detrivore cucumber would need something as drastic as its body being torn or pierced before it will emit toxins.
Go get yourself a detrivore sea cucumber and your sand will never be dirty again. Mine even cleans the live rocks.