Flavio,

You did not mention what, if any, kind of filter systen you have.  If you do
not have one (and actually, even if you do) your best bet is to change 20%
of the water weekly (sea water siphoned out and thrown away, more sea water
added to replace it).  You really should measure the salinity, however, if
you collect as far away from the freshwater source as you can, you should
still get seawater of sufficient salinity to keep your critters alive.  You
should also replace the rocks regularly as I doubt if you have sufficient
light to keep themn going.  It would be best to use each rock for a week or
so and then replace it where you found it.  That way it can be re-inhabited
by the algae, sponges, etc. that were there originally.

Tom Eichhorst in New Mexico, USA