Well that just plain isn't the case. Up and down are relative directions, not absolutes, and as fish are unable to think like this, we (humans) are the only ones able to define "up" or "down".
Even for us they are relative, but generally speaking on earth they are defined either by gravity or by feet and head on a human body standing up. Either way, whatever way a fish swims, it's not upside down by our definitions. And however you chose to define it, we still class it by our definition, not by "what might we think if we were a fish capable of human levels of thought?".
And as to front/back... well that's theoretially described by the direction of the animal's normal movement. But regardless of that, again if you are defining a fish, it can only be defined by our grammar, not by their world.
Even for us they are relative, but generally speaking on earth they are defined either by gravity or by feet and head on a human body standing up. Either way, whatever way a fish swims, it's not upside down by our definitions. And however you chose to define it, we still class it by our definition, not by "what might we think if we were a fish capable of human levels of thought?".
And as to front/back... well that's theoretially described by the direction of the animal's normal movement. But regardless of that, again if you are defining a fish, it can only be defined by our grammar, not by their world.