Losing salinity

Question
Hello, and thanks for any help you can give me.  I'm new to the marine world, and I just started my first salt water aquarium.  I've had fresh water for many years, and was a little *petrified* of salt water, but figured what the hay, might as well.  :)

Anyways, I kinda cheated on my first one, but it seems to be going okay so far.  I live on the coast, so after I got my aquarium set up (29 gal with 60 gal filter/ heater/ wavemaker, protein skimmer, and 18" bubble stick) I hit up the beach for some "live" stuff.  Using sand and a few rocks all immersed in water, I took it home and tossed it in.  Wanted to wait on fish, but my hubby surprised me with a few free fish...so now we have a running aquarium...been testing like crazy to catch any spikes and to see how everything is doing.  We've already got an abundance of small life (worms, baby crabs for now, and sea fleas, snails, ect.)  And the fish seem to be doing okay...but I'm losing salinity.  Is this because of my filter or what.  I don't want to add too much salt at one time, but at the same time I'm under where I started from on salt content and SG.  Brought it up a little tonight, but I'm still low.  

If I keep up the testing and dosing, will this eventually stop, or did I buy the wrong filter.  I know that it said on the box that it was marine able.  Filter is a TetraTec 300.

Answer
Hi Edna,

    This is a new one. Usually as your water evaporates your salinity will increase as only water will evaporate leaving a higher salt content. I do not have any idea why your salinity level would decrease. Are you reading your hydrometer correctly? Nothing in your system should be causing a loss of salt unless there is a leak somewhere and you are replacing water loss with fresh water. As your water evaporates you should replace it with treated freshwater and the salt content would remain constant. The only way to remove salt from the system is if saltwater was leaking out somewhere. Please check your readings again and get back to me.