The portion on the left is natural fresh mahi fillet and the one on the right is CO2 treated (gassed). When viewing the product in the field it is best to look at the bone side of the fillet or portion. The skin side can often be quite similar. In addition you will notice that the whole body of the portion of the treated piece sort of glows pink. Whereas the natural portion is much softer. No pun intended, but the natural piece looks natural.
The taste profile can vary based on the timing and quality of the gas process.
But it is safe to say that the CO2 has a distinct aftertaste. It is best described as a slight metal taste. This can vary from slight to overwhelming.
This process is used heavy in tuna production for frozen portions. Those candy apple colored tuna items are all treated. The processors can take very old downgraded product that is completely brown and “bring it back to life”.
The bottom line is the taste profile which can’t be tested until tasted. Fresh is always better.