Yes, cedar is NOT a good idea for aquariums. The phytochemicals in cedar wood (ALL TYPES -ex:Western Red, Eastern White, Atlantic, Incense, etc...) are technically toxic. That's why they are not used for small rodents, parrots, ferrets and so on. They use small amount of cedar chips in dog bed to kill fleas, and cedar moth balls to kill moths. Obviously they are being used for their toxic properties. In both cases they are being used in small amounts to kill insects. Imagine now putting that wood (the oil is the toxic portion) into an aquarium where it will leach into the water in an enclosed space, with a filter that will be ineffective at removing it (neither mechanical filtration NOR biological filtration will remove it).
I know people have done it, but why take the chance. There are some cockroaches that become immune to the poison specifically designed to kill them, but that still doesn't mean it's not poison... Just my thoughts.