boil the hell out of it (your wife will love you for the smell..... dont ask how i know about prepping skullsfor fish tanks) to make sure all the marrow has gone and any bits of brain. then varnish it with quite a few layers to seal it all. then pop it in the tank. but even after all that there will be something inside that will rot and fook up your water.
I remember reading somewhere about using antlers in the aquarium, and how it can cause issues. Not sure if your skull has horn/antlers but worth taking into consideration and researching a bit