I think you may have a outbreak of Ich!
Treat with special chemical's and read the label on it as to how much to dose fish with it.
You will have cloudy water as your filter hasnt filertaion all of the water.
And also you wouldnt have much bios which is a good thing.
Hook the dead the fish out , leave the tank alone.
Wait for 2 weeks then things should of cleared.
Also if you havent already buy a Api Liquid Test Kit
It is £25 but will help you massivly.
It is a must have really.
If your Ammonia And Nitrate Level's are high then you should partial water changes every few days.
Buy a gravel vacum cleaner!
And always remember when putting in new fish water add the correct amount of "Conditiner" Which takes the chroline out so your fish will stay alive.
Dont buy any fish for about a month!
You should of waited a week then bought some hardly fish i.e mollies platys !
But dont worry we all make mistakes when we first start.
Just keep trying and dont give up!
I have been keeping fish for just over 3 months and i still havent got my fish tank as to how i want it yet.
Its trial and error.
Until you have a few years experence you will make mistakes!
Dont worry about it
Hope i helped and sorry about my grammer , i am just a school boy!