New 55 Gallon, Fish keep dying

Question
We just setup a new 55 gallon tank.  Only one of the fish from our previous tank survived our move(Jack Dempsey-2 years old). He appears to be very healthy. We have added a Turqouise Severum, Midas Cichlid, plecostomos, and an Oscar.  We have lost one plecostomos and now two Oscars.  Initially we believed that we had a fungus issue and have been treating the tank accordinly, but the first Oscar died.  Then the plecostomos died(however this may be to a lack of green algae in the tank yet, so we are adding tablets now on a daily basis).  Now the replacement Oscar has died,  he was very white in color and almost had a skim all over him, like he had been dead for days, however I just fed him this morning and he was fine.  I have noticed that the Midas is starting to be a bully.  Is it the midas causing the problems or something else?  All Water tests check out normal?

Answer
GET RID OF SOME FISH AS FAST AS YOU CAN!

You are EXTREMELY overstocked. You should only put 1 oscar with NO tankmates in a 55 gallon tank, let alone all of the other SA cichlids.

It is impossible for your levels to check it out, because i dont belive you cycled your new tank. Your fish are dying from ammonia poisoning.

You need to research on how to cycle a tank.

ALWAYS do your research BEFOREHAND, to prevent mistakes and mishaps.