Fish Killing Factory?

I don't understand what's going on. I'm suddenly killing a lot of fish... and wasting a lot of money.

About a week and a half ago, I got two new fish from a new LFS that I wanted to try. That was on Saturday. On Sunday, I had found one of them dead. This particular LFS had a 48 hour guarantee, which is why I wanted to try them out in the first place. So, I brought it back. They didn't have any of that kind left, and there weren't any at that dollar level that I liked. So, I ended up replacing it with two smaller, cheaper fish. On Monday, I found one of those two new ones dead in the tank. Livid, I went back to the LFS, and they gave me a store credit for that fish. So, now I've got two fish in the QT, and some store credit. Neither fish is eating very much, and both seem very skittish.

Cut to mid-week. There seems to be some sort of bite mark or something on one of the new fish. The edges of the missing area are all white (on a red fish, it stands out a lot). On Sunday, I decide that it's getting worse, so it wasn't a bite from the other fish. He tells me it's either a skin condition or a fungus. Gives me some special juice, and I go on my merry way... or so I think.

Cut to Monday. The second of the smaller fish in the QT is dead. When I pulled it out of the tank, it looks like its fins are all worn away. Now, I'm diagnosing it as fin rot. The medication does not seem to be working, and the last fish in the QT seems to be getting worse.

Cut to today - Wednesday. I came home, and I couldn't find Pinnochio right away in the display tank. I looked everywhere... or so I thought. Where I didn't look was on the floor behind the tank. Yes, folks, that's right! He jumped out of the tank. Yet another dead fish. I go downstairs, and the QT'd fish's fin seems worse. I'm definitely thinking fin rot at this point. I don't know how long it is going to live, but I am assuming not much longer.

So, in the course of about a week and a half, I have had the distinct pleasure of finding four dead fish, and will likely make it a fifth in short order. What the hell am I doing wrong? This is really discouraging from going any further with this hobby. I have slowed down EVERYTHING. I can't go any slower than I am already doing. I am definitely not buying any more fish until after my exam in a few weeks, but still, this is really disconcerting. Not only am I wasting my time and money, but I'm seemingly killing poor living creatures.

I just don't know...