Well, for healthy water, the water needs to be changed 50% weekly, more if there is an infection present.
Also, define normal?
without knowing the exact water parameters, we can't know exactly what is going on. If you don't have a testing kit, take a sample of your water to a local fish store. They will ussually test your water for free. They tend to use the testing strips, which aren't that accurate, but it would help us know the roundabout numbers for your tank. protruding scales are an indication of an internal infection usually.