Sorry for your loss Dipali.
I see a number of issues...
First that you added the white Discus without proper quarantine period.
Not knowing your tank water perimeters leaves you with unknow answers which may have contributed to your fish loss.
I would check your heater condition as you mentioned possible electrical shock.
You say tank has been running for 4 months? Did it contain the 19 other discus from the start or have you gradually added discus throughout the 4 month period.
Storage tank outside? is it food safe and does it get direct sunlight which may leach toxins into the water your putting into your aquarium.
...Ralph