In general, fish naming is a mess, and fish are often mislabeled. But the term turbot is particularly misleading as there are several kinds of fish that go by that name.
Kosher turbots may include fish known as “Pacific turbots” (Pleuronichthys species), “Curlfin turbots” or sole (Pleuronichthys decurrens), “Diamond turbots” (Hypsopsetta guttulata), and “Greenland turbots” or halibut (Reinhardtius hippoglossoides).
Conversely “European turbots” (Scophthalmus maximus or Psetta maximus) are not kosher, and are found all over the world, not just in Europe.
So what are you to do?
If you catch what you believe to be a turbot (or buy one whole) and you see the fins and scales, cook or fry it in your kosher kitchen and enjoy. The same applies if you pick up some fish labeled “turbot” at a certified kosher fish shop or in a sealed package bearing kosher certification.
If you are purchasing fresh fish from a non-kosher establishment, ask to see the skin still on the fish to make sure there are actual scales, and then ask the store employee to clean the knife well and cut your fish on a fresh sheet of paper. You should then carefully wash the part of the fish that has been cut in water that is no warmer than room temperature.
Unlike meat or fowl, fish does not have to be slaughtered or salted, so nothing more must be done to make your fish kosher.
Hearty appetite!
If they can not swim above the surface enough to be able to rest above thr water, and their face attached straight way according gravity, what is the point of having fin which is just like other unkosher fishes simply living on the surface? And if scales can not be even detected by eyes and not even be felt when eating, how do you regard they have these features, rather than its feature is marely left in the process of degenerated?
I can't believe Halibut is regarded as Kosher.
Where is the basis of ruling specifically on this fish can you please direct me?
The Torah only says that fish needs to have fins and scales, without any other qualifications. As such, if it can be determined that a fish indeed has both, or had it at some point, then there is no reason for say it isn't kosher.