I found 2 mistakes on your setup. First, you should never use regular freshwater. You need to go ahead and use a filter so that you can remove the minerals that are in tap water. The filter will remove the chlorine as well. These minerals cause algae to grow in saltwater tank. Dechlorinator can build up over time and can only be removed by doing water changes.
Also, you should mix your salt with the water in a bucket before adding it to your tank. Purchase a small air pump with an air stone and aerate the water in the bucket.
There are 2 reasons to aerate your water before you add it to your tank. 1st... Water out of the tap has almost no oxygen in it. If you were to pour this directly into your tank, you could actually suffocate your fish. 2nd. When you measure your Specific Gravity, your readings will be off if you do not aerate. This is because there is no dissolved gas in the water. As oxygen is absorbed by the water, the SG will be increase. If you mix your salt to 1.023 before you aerate the water, later, when the water has absorbed oxygen by cycling through your filter system, your final SG will actually be higher than where you set it. You can not mix salt and water properly if you don't first have the water aerated because the SG will increase once the water absorbs oxygen.
There should not be any problem with your sand. The declorinator will remove all chlorine. If any has been absorbed by the sand, after a few days, the dechlorinator will remove it.
You could buy some live rock now if you want to, but if you can, it would be better to empty out your water and replace it with filtered water like reverse osmosis, de-ionised, or distilled water. If you can't do this right now, then that's ok. It will still work but it may not be the best setup and can lead to algae problems in the future.
If you buy live rock now, you will have a lot of die off. It would be better to cycle your tank with a simple frozen shrimp instead. But if you really want to go with the live rock now, it can be done but expect some algae problems and also for most of the live rock to die. This is because with a new tank, your biological cycle hasn't yet started. Whatever you put in there will go through some harsh changes while your tank is "cycling". If you put live rock in right now, most of the invertebrates within it will die because they can't handle the high ammonia levels that you are going to get. As your live rock dies off it will provide ample nutrition for nusiance algae. You can get rid of it over time, but you would save yourself a lot of hassle by skipping the live rock until your tank has cycled.
Like I said, you could cycle your tank by using a frozen shrimp. Throw it in there and let it float around for about a month. Keep testing your water and when your nitrite levels finally go down, your tank has cycled. Do a complete water change with filtered water and you are ready to begin adding in your livestock. Add things slowly and give your system time to adjust to each thing you add.
