Hi Hooi
Guess its typo that the tank is 4ft.
In your first post, that the corals are surviving does not mean anything as coral might be dieing slowly. Fishes will be obvious as they stop breathing and goes into a RIP stage.
Here's my 2 cents:
1. Check for electricity leakage into the tank as suggest by LAW bro. This normally will kill fishes not corals. Corals tends to grow faster with certain amount of electricity as its proven in Indonesia reef which they use this method to salvage a dieing reef.
2. Let the tank settle for a couple weeks which FuEL bro had commented.
3. Check for predator in the tank like mantis shrimp. It may be hiding and killing small fishes like damsel, clowns.....etc.
The best way to have more accurate suggestion and comments from senior bros in this forum (exclude me as I am also learning), a picture tells a thousand words. Get your friend to post a picture of the tank and the equipment like sump, skimmer...etc.
Lots of us are willing to share only if we know the "full picture".