Why did 3 fish just die in a week and a half?

Personally I would re-check your parameter against another kit or at a LFS. ;D

 

Really sorry to here about your loss. Im a person thats not apposed to buying fish that will out grow my tank if you have a new home they can go to or are planning to get a bigger tank in the near future. I hope you can find your problem. I do agree it sounds kinda like a hitchhiker.

 

What size is the Clown Tang?

Why are your nitrates zero?

 

Your temp swing is huge i agree. Im usually within 77-79 without much problem. I have a fan to help me out during the days that are really hot. I dont think its the reason for your fish dieing but it will put un-needed stress on your entire tank. Also maybe you need a diff heater if allows you temp to drop to much before kicking in (unless your problem is getting to warm then a fan is the answer)

Keep us posted on what happens over the next few days. The people on this forum are wicked and are such a great help that im sure your problem can be resolved.

How long has the tank been running for? I agree with ingtar_shinowa and Corailline go to your lfs (local fish store) and get them to test your water.

 

Haha, I saw that too, it should read two clown FISH. I hope. A Clown Tang is a serious fish that grows fast and can be extremely aggressive. They are no joke.