I'm sure others who are more knowledgeable than I will also chime in. Are you buying premixed salt water or do you have a system of making your own? I buy mine premixed from the pet store, but I have a smaller tank and the 5 gallon box for 10 bucks lasts me about 3 weeks.
For a 28 gallon tank you'll want to unplug everything from the wall once a week, scoop out about 4-6 gallons and replace it with new salt water. You'll figure out what percentage water change works for your tank as you test your levels and see what keeps it stable. You will have evaporation throughout the week so you'll want to keep a jug of reverse osmosis water (non salted) by the tank and dump some in every few days.The water evaporates but the salt does not. So dumping salt water in will just make the water saltier and saltier.
You have a biocube, do you have the filter in there? The bioballs? If so you'll want to watch the nitrates (well, you'll always want to watch the nitrates but especially if you have the bioballs and a filter. If you do have the bioballs you might want to start removing them slowing during water changes, maybe a few a week. And if you have the stock filter and you want to keep using it, it'll have to be rinsed in old tank water once a week and replaced once a month. I got rid of mine and just keep some filter floss on the top rack of the middle chamber and I pitch it every few days and replace with new.
You will need a testing kit. It doesn't sound like you have one, they sell them cheap on amazon, api liquid test kits are great, I got mine for 17 bucks with free shipping. You should check your levels weekly. It's kinda fun too, you feel like a chemist :). When you post questions people are going to want to know your specific levels as these can be the cause of your issues. Like, if your nitrates rise, like mine, you need to do a water change or adjust something to keep your tank healthy. So people want to see numbers, like nitrates are at 10, not "my levels are fine" But you'll especially want to know your levels as you've just moved your tank and moving all the sand can start a cycle. (others will know more about this than I) and a cycle can kill fish and larger, more frequent water changes til everything stablizes may be necessary, but, again, you will not know this without testing.
Do you have corals in your tank? If not, the salinity is a bit high, but, this will come down over time as you are changing it out with 1.022 premixed salt water and the fresh water top offs.
I hate to keep asking this, but, do you have the little flap on your tank open? the one on the front of the lid? It looks like it should be shut but it shouldn't.
Congrats on your purchase, the biocube is a fun tank and I think you're really going to like it