Rats adapt to vision loss because they have terrible vision to begin with. It looks like a cataract but it also could be glaucoma which can be painful. 18 months is terribly early to develop cataracts. Is he blinking painfully, is there excess porphryin, is he rubbing at it?
As for his care, he doesn't need baths (they groom constantly), or vitamins (as these should be supplied by his balanced diet alone) and if you are putting vitamins in his water, you are wasting your money as they become ineffective in light and in 24 hours, and the bottle needs to be changed and scrubbed every day so bacteria doesn't develop.
What else has changed? A sudden change in behaviour usually can mean pain or illness.