Sand is naturally good for bottom-dwelling fish ( corydoras, loaches, etc... ) If you do add sand, then I would suggest a thin layer... ( around 1 - 3cm )
If you want to go for the cheap option ( which is to buy play sand, silver sand, etc ) then you usually have to wash it a lot! i just added play sand to some of my tanks and I simply put a pile of sand in a large bowl, added water and then just literally stirred the sand about. I then tipped the ( which will be horrible ) water down the sink and then added some more water and do the same process until the water in the bowl is more or less clear...
Cheers...