Take your time and research look into refractometers, liquid test kits, aquaclear fuge, and ATO. An ATO will make keeping the tank stable when it comes to toping off, refractometer for testing the salinty
Paint the back of the tank that plastic film is a nightmare in the making.
Research and ask question here and at the LFS. You will find knowledge will be the best ammo against the advice of LFS. You find they will try and make a sale over the best interest of the live stock or customer.