Why did my fishes die?

I got a fish tank for Christmas '09. I got 2 glofish, 1 neon tetra, and 1 zebra danio. I already had a beta. I put them all in this tank together after waiting TWO days for the water to get room temperature and followed ALL of the directions. About 4 days later, my neon tetra disappeared. Yes, it disappeared, I looked all around the tank. Then, about a month later, before changing any water, one of my glofish disappeared. I changed the water. About 4 months later, with routine water changes, my other glofish dies. What am I doing wrong?! I love my fishes, and don't want them to die...