I have a black concrete fountain which has white buildup,like salt,on the outside. Does anyone know what I can use to remove the white and return the fountain to original black colour?
This salts build up is relatively normal for stone,concrete and pottery outdoors, particularly on anything that involves water. Where you live might make a difference, because what the salts are comprised of will differ - in the UK, they'll largely be limescale staining/build up. You can try mixing 2 cups of vinegar with 2 tablespoons of washing up liquid (dish soap) in a bucket of warm water, wet the concrete first, then scrub it with the solution using a stiff brush. Because the concrete is black (presumably dyed at the mixing stage) if the staining is within the concrete as well as sitting on the exterior, this might not be 100% successful. Be sure to rinse it off properly in case some of it gets into the interior of the fountain, or you'll finish up with soapy froth when you turn the fountain back on. Link here, although it's aimed at cleaning a concrete floor rather than a fountain http://www.howtocleanstuff.net/how-to-remove-salt-stains-and-residue-from-concrete/
This may shift some of it, but, as it's a fountain, it's likely to be a problem ongoing, so I'd try that first, see what it looks like when it's done, and then buy black exterior concrete paint and accept it will need painting, probably annually, or maybe every other year or so, after a good scrub each time. Ronseal make doorstep paints in black (if you're in the UK) but probably you can get black concrete paint for exterior surfaces wherever you are.