new fish

I got a new betta, a red one since we've had bad luck with purple and blue ones. Stupid, I know. He is very small. I hope that means he's a baby and will get bigger. He's like half the size of Betta Boy II. I also picked up a couple more albino corydoras. I was talking to the guy who was waiting patiently for me a to pick a betta (they are all so pretty! Why won't they get along? Imagine a 55 gallon tank filled with bettas!) about the problem I have with my corys getting fin rot and having their tails fall off. He showed me some different medicine although he said the medicine I have should work.
So I get home, put the two bags into the tank to float and go about my business. I come back in, notice for the first time the miniature size of Betta Boy III, and then notice that one of the corys is ALREADY missing his tail. Now I know gas is $3.50 a gallon and the fish were buy one get one free but it's the POINT. I drove back up there and got a new fish.
I also picked up something to put into the tank, some kind of good bacteria, that's supposed to lesson the shock to the tank of putting in new fish and also for water changes. (Did one of those yesterday too.)