Musk Turtle Problem

[08naca1]

I am the owner of two 5 year old Musk Turltes and i have recently noticed that one of them has very swollen eyes (both eyes). I had read that this could either be a result of water quality or vitamin A deficiency but he is on a supplemented diet which is varied too! I recently tested the water and change it regularly so i dont feel it is related to this.
I have become increasingly concerned that it may be something more sinister as over the last 24 hours he appears to be having difficulty swimming downwards and appears very buoyant. He also is resting for long periods and then swimming very erratically and repeatedly in circles.
I have separated him from the other turtle in a tank with less water so he can get up to the surface and breathe and not injure himself on items in the tank when he swims and spins. He doesnt have any mucus and is not coughing or wheezing but is regularly coming up for air and bending his head in the most bizarre angles!
I am very worried as this hasnt happened before! Hes been to the vet but they suggested that it was simply a vitamin A deficiency however i feel as though it s something more sinister!
I was wondering if anyone has any advice on what i should do or what the problem may be!

Many thanks,

Natalie