Try testing out the water for ph spike, i'm certainly sure your too low at the moment. Do you have oxygen going through the tank? This is what i would do, get some sea shells and spread a thin layer not too much but just enough to cover the bottom layer. This will help you to stablised the ph spike, while your tank cycles in a few weeks time. Change 30-40% water weekly and 50% water fornightly, check your ph scale and other values of properties in tank such as ammonia, nitrate, nitrite when you change water, but i can assure you its the ph spike which is killing your fish.