Wouldnt recommend. In addition to being a "soft" wood and like to decay faster in your tank, cedar wood has oils in it that take years of dying to get out. The only way youd know for sure if it was full cured would be to cut it in half. Cedars have a core that will stay oily and aromatic for years.
There was a cedar tree here when we moved here 20 years ago. It was in a square brick place between and in front of the two joined/attached garages. Through the years I've planted several things where it was, they all died. I was told the only way would be to dig all the dirt out and replace it. Sorry, it's a beautiful piece of wood.