New tank and snail problems

Hi everyone! So I just got into this hobby about a month and a half ago and it has been pretty rough. I didn't do any research before and just kinda jumped in with the help of a bad fish store. So I started off with a 10 gallon Jebo (which apparently isn't good for saltwater). I was advised to buy a clownfish and a damsel within 24 hours of buying my tank and about a month in was advised to buy an anemone which ended up doing very badly and is no longer in my tank. I found a new LFS who I trust very much (they don't let me buy things that my tank can't handle so I know they're good haha). I bought two snails (turbo and margarita) and a cleaner shrimp. The turbo died within like 24 hours. I think it was because I didn't take him out of the bag properly (I just kinda pulled him out) and now the little margarita is on the bottom upside down (maybe dead also?). So my first question is, why are my snails dying? All of my levels are where they should be and the temperature is also stable and around 79C. The fish are fine. I don't understand. Could it be something to do with the shrimp? I've read some forums about shrimp eating snails. My second question is what kind of new tank should I get? I am planning on getting a 30 gallon. The biocube looks really good to me, but I'm new at this so I wanted some other opinions. Also, does anyone have any general recommendations for me? I want to be able to do well in this hobby! I will not give up!