Possibly solved.
After seeing how NYGC's aquarium failed as I did a long time ago with freshwater aquarium in the States, I suspected for some unknown reason, the water became really bad for the fishes. But I didn't say anything, because it had always been a question to me, not a certainty.
New fish just kept dying within days.
Some staffs bought an API Freshwater Master Test Kit (around $25 on Ebay) to test the water and all the readings were way off. Now, the fishes are fine.