Terrapin shell turning yellow.

Hi,

My Terrapin shell is turning yellow. My Terrapin came home with us about 3 months ago. They are eating well, small dried prawns that we bought in a pet home.
We change the water every 5 days. May i know also if it is normal to change that often. We fixed a small pump, but still the water smell after 5 days. And may i also ask if we need to add some salt in the water?

Thank you.