Thanks for the information,
The tank is a previous set-up with the same components i.e. the same 3 small fish etc. From my knowledge these fish have not had any critical health issues in the past 3 months, while in the same tank. This is given that the tank has a weekly 15% pwc
The puffer is a baby spotted puffer fish about 2.5cm long
The two clowns are about 1.5cm in size.
Another point to make is that 5 days ago I moved the tank from it's location. This includes a change in some corals and live rocks and more live gravel.
Could it be that the tank move disrupted the natural good bacteria growing in the live rocks, live gravel?
Also do you know if the PH levels can be affected by live gravel?