One of the problems with living in Bangkok is that the staff don't often speak English and can't tell me exactly which variety of fish I am buying -- so I will probably have to post a photo for you to know which Blennie I have. It is not a Midas, though.
In the evening (when lights are out) it sleeps on the bottom, but the rest of the time it is always on the top.. and seems like it almost having trouble breathing and its mouth is very near the surface. I thought it would die, to be honest, but after 2 days it is still the same.
Thanks for your suggestions!