When the water evaporates it leaves the salt behind. If you don't use 'top up water' and just add salt water you'll concentrate more and more saline water till stuff starts dying.
Don't add any more large animals , fish or inverts to this tank!. Has this been set up for long - you need a lot of patience for this. Where do you get your stuff is it a good shop, or is it a chain?
The live sand and live rock is good, though you could easily use another 10 -20 lbs of rock. You have to make sure it is cured , and smells good else when you put it in dying stuff will pollute and kill everything you have.
A skimmer is a good investment and I would recommend a hang on prizm or better a more expensive remora-c. You can get very cheap skimmers but they are hard to get working well. They are very good at removing pollutants. You will find life very difficult without one as you'll need to do large , expensive water changes at least once a week.
Please talk to as many people as you can about what you're doing - I think you're on the edge of obliterating your tank already. You ned lots of patience. Please, please, please, no more fish or big inverts till your tank is more stable - this may take a few months.
Do you know anyone else locally who can show you what to do? Not people who've had a tank going great for 2 0r 3 months, people who have stable 1 year old plus tanks. They can help you out more than internet questions ever will.
Read more books - 'The Conscientious Marine Aquarist ' is very good , look at Bob Fenners website wetwebmedia.com.
Good luck