I prefer around 30% weekly with larger fish, there can be several causes of cloudy eye, as stated ammonia or nitrites can do it, if that is the cause good water will let it heal on it's own. If it is bacterial you need to keep up with the water changes, lowering the light level a little and giving a varied diet helpsprevent it by raising resistance but antibiotics such as amoxycillan or tetracycline can be used to treat it.