Newbie with Musk problem

[SquirtleD]

Hello Everyone,

I am new to the site and keeping a turtle and unfortunately need help already

I have rehomed a 2 year old Male Musk who seemed to have settled in perfectly however a few days in and he seems to have his third eyelid/membrane (I may be wrong) permanently shut when in water which is affecting his vision.

He has a shell length of approx. 3 inches and is in a 100 litre tank with good filter, basking area and has a small baby pleco as a tank mate.

I first thought it may be the UV light was too strong so this has been turned off two days but no change. Water levels are all ok but I have done a 50% water change just in case using an additive to the tap water to de-chlorinate.

Last night I tried to clean the eyes using so boiled water that was left to cool first using a cotton bud and there was no crust, gunk etc. but after being out of the water for a few minutes his eyes appeared to go back to normal.

My only conclusion to this is that it must be the water. I am using a Fluval water treatment I have previously been using for my tropical fish tank without issue but could this be the cause? Also he was previously fed on shrimp and blood worm and for the last few days he has been on pellets which say they have added vitamins however he has been eating and not lost his appetite.

Thanks in advance, any help appreciated.