Ouch!! What kind of snails are they? Some of those snails will not hesitate to over populate a tank! Like the Malaysian trumpet snails, I know a guy local to my area works at a pet store and sells these year around on Craigslist just to keep his tank from over populating, and I have refrained from buying them with this fear in mind!
The fish you had die, it is quite often a toss up when buying fish and most times you want to find the healthiest. You also want to check the stores water when you get home, I had one pet store my fish would die over and over again, I never had one survive from the specific store. So I went in and got a fish just to test their water and I found it was so acidic with a reading of 6.2!!!! So I flipped and emailed them and everything because a .2 change or more in PH within 24 hours can hurt / kill a fish, meaning if you got one in acidic water it would take a week or more to properly acclimate. This store only offered a 48 hour warranty, so I stopped shopping there period. Only shop if there is at least a 2 week period to return the fish, bring in your water and let them test it. Most fish stores offer this warranty because they know how fish can die or be a toss up. I have bought many that did not survive the night.
As far as your missing tail, your other fish may be a bully to it. And if that happens, the tail will heal but it wont if the other bullies it. The more damaged it gets the more the other will bully it and the more it cannot evade the bully. It is like one fish slowly pounding the other to death so it can eat it. It is common, but it only gets worse from there. There may be a mis-balance somewhere causing it to feel flustered. Are they all males? If you have all one sex, you need all female, if it is males, you need a good 2-3 females per male. This will reduce tension.
Also make sure there are decorations for the fish to evade and hide with. That plant you tossed was probably a help to the hurt fish because it can dodge and hide behind plants... But as far as the bully fish, I would keep an eye on them. The weaker fish get more of the beef because they are already beat up. Fish will kick other fish while they are down, and that is what they prefer to do. Your hurt fish will heal if it is left alone by the bully fish... That is split / damaged fins, if the fin is missing entirely it may not grow back completely, or if it did it would take the length of the fish's existence to make progress with the growth...