I think that you are missing the point. Online vendors use this as a practice to deceive unwary buyers, that cannot see the fish in person.
This is not news to anyone that has been involved in the hobby for more than a few minutes. Just take a look at how many online vendors in SE Asia promote their "red" fish. This has been a common practice for many years, and still is. In person, such as at the market you mentioned in Bangkok, it would be quite obvious to the buyer as to what type of lighting etc is being used, so no real point in attempting deception, better to show the fish in person, exactly as it is in real life, under normal lighting.
Armutlu, there are many commercial pellets available that contain far higher levels of Astaxanthin that one will typically find in salmon, shrimp, etc. Some of those brands have already been mentioned, Hikari Blood Red Parrot+, Grand Sumo Red, and I will add New Life Spectrum Ultra Red. Those foods will contain FAR higher levels of Astaxanthin, along with numerous other color enhancers, and nutrients, than one will get from feeding seafood. If your fish still isn't red enough for you, put a NAN light over it. lol