Since there's no fish in there, you *could* try using a flocculant (there's
one specifically for use in aquariums called 'filter clear' by Interpet) to
see if it's tiny particles in the water causing the cloudiness. It'll cause
them to clump together, and it's good to do before the fish go in, in case
it irritates the fishes gills at all. Once the particles are larger, they
will either settle out to the substrate, or be trapped in the filter. You
can normally see it working within an hour of it being added, but it might
take longer if the particles are very tiny. Alternatively, if the gravel
was new, it might just be dust from that, which the filterclear would also
take care of.

I find my tanks are normally cloudy for a few days after being first set up,
but I go straight in and cycle with fish. Normally by the end of week one,
the water is crystal clear again (and sometimes within 4 days).

Good luck!

Velvet