I second the idea of pressure washing aquarium wood. I was amazed at how effective it was the first time I tried it. If it knocks off a part of the wood, that part was probably decayed and shouldn't have been in the tank anyway. Obviously you don't want to use really high pressure, like 2000 psi, for this, but any pressure that is safe on a car shouldn't be too much for the wood. I have a cheap pressure washer for wood decking, that I use, and I just keep the nozzle a foot or so from the wood to keep from over pressuring it.
By the way, that is a great chunk of wood, worth a lot of effort to get it usable in the tank.