did a little more reading on cedar, it can be used but you want to be absolutely sure that it is fully dry all the way through by cutting it in half (which you've done i believe) and looking to see if it is fully dry or not. if its not then it would still have sap inside, and sap can be very harmful to your fish.
if there is no sap then yes, will probably have to tie it to something so it will sink. other then that, give it a really good scrub, and boil it if you can. i would not recommend soaking it in bleach as the wood will absorb the bleach.
It would dry it out for sure, just don't let it catch fire.
I honestly wouldn't use any type of wood that is identified as red cedar in an aquarium. Red cedar has a natural antifungal in the wood so it does not rot as quickly but also contains a natural pesticide. It doesn't sink as quickly because it doesn't absorb the water like other types. That is why it is common in porches, lawn furniture, etc. That is the difference between cedar and red cedar.