Are you using a chiller or fan?.. If you are using a fan the drop in water level could be due to evaporation... then just top up fresh water
If the drop in water level is due to the skimmer, i think you should replace it with salt water.
I know its quite hard to maintain the parameters since the volume of water in the tank is small.
I think if there is a small change in salinity, gradually over a long period of time, it wouldn't be too harmful
Senior reefers, correct me if i am wrong