Missing tail

Question
I am a brand new fish owner.  I have a 2 gallon fluval spec tank with a heater.  I got 3 melon bars and one bottom feeder fish (I forget what it's called).  One of the melon bars is a super big bully so I put a mirror next to the tank for one day and now that fish seems to be calmer.  The smallest melon bar is now missing his entire tail! Is he gonna be okay? (It's only been a week since I got these guys!  I feel like a fish failure!)

Answer
Bea,
I am assuming you have a red melon discus fish. The tank you have now can only house one Betta. The fish you have are too big for the tank. I am also assuming you have a Pleco, which is the "bottom feeder." If it is a pleco, he too cannot be in that tank. These fish need very large tanks because they get very large. If it is not a discus fish, can you upload a picture so that I know what we are talking about. The fish are acting this way because they are territorial and they do not have enough room to be that way.