I've got a pretty similar problem with my aquarium (Total beginner with forums and aquariums this, so sorry )....

Have been using water testing strips and everything seems fine, fish all healthy and happy etc.

The water takes on a 'milky' appearance even after twice weekly 25% water change. The filter is one in the top of the tank (not under gravel type) with bio rings and white filter wool and carbon sponges. Glass is cleaned each time I change water as described above.

I've tried everything just to have a nice pristine looking tank and now I'm lost for answers hoping somebody can give me some answers. Much appreciated if anyone can help?