How to make the boxfish look fat?

[sinhotang]

So my white barred boxfish eat 4 times a day and it is so healthy but it looks so thin that its belly is sunk.I think this happen to other boxfishs?However,in this video the white barred boxfish look extremely fat that their belly are swelling just like a goldfish.So i want to know how to make my boxfish fat like those in the video.Anyone?
https://www.youtube.com/watch?v=TioFxtxDf3o

 

[moriarty]

What temperature are you keeping your boxfish at? The reason that I ask is that Anoplocapros lenticularis is a temperate species. You are likely aware of this, and temperature may not even be at play here, but in the interest of being thorough... One of the "symptoms" of being kept at elevated temperatures will, among other things, be an increased metabolism. You mentioned that you have been feeding 4X daily, and the fish still has a sunken appearance. There are of course other possibilities (parasitic... etc...), but I thought I'd start with the most obvious