So as the title says, I'm very new to anything aquarium-based. I had a Betta in a 5G tank 20 years ago, that's it.
RIP Eeyore

At any rate, I have a 3 gallon tank with aragonite sand for the last 3 weeks. However, just yesterday I learned (from this site) that "Brackish Salt" is not a good way to up the salinity. So I removed most of the water (say about 90-95% water change) with the proper water and marine salt. SG is about 1.011 or 1.012.
I have ordered snails, a macroalgae, and a mini mystery algae ball. I did not order the shrimp yet since the tank is not established yet. So it should come in mid-week next week?
I have 2 questions, though:
1) Will the snails be ok? I see some stuff floating on the water's surface, I'm assuming that's some sort of bacteria from the leftover water I had when doing the water change. The tank is not established, so can the snails survive?
2) I keep on reading about putting in calcium carbonate, etc. into the water. But I also heard that argonite sand IS calcium carbonate. Is this enough? Would I need to add anything else to the tank?
Any help would be appreciated! Any extra tips also would be great

Take care!