Chinese algae eater needs to go. They are very aggressive fish that will attack your other fish. They will take out pieces, scales, slime coat, or even just batter your fish.
Honestly, I think that's what happened to your zebra danio. This is a common thing you see on the forums and it happened to one of my fish when I was a newbie. They need to be in a large species-only tank.
The Siamese algae eater's numbers need to be upped. They are a schooling fish and will be much happier and healthier in a group.
The blue fish is like a platy or molly. Some sort of livebearer. I have no experience with them whatsoever, so I can't help you out there.
The rainbow shark should be fine, needs hiding spots though. They can become territorial over a particular rock or cave and attack others that come near, so watch him.
The goldfish, like I said, you need to contact a LFS or find a home to take them in. Or upgrade to a much, much larger tank. I would recommend 200g+
And a shark? Could you post a picture of that?