MrTusk is right. Panther grouper grows far too large and active for almost all home aquariums. It will reach over 20" easily and is really only suitable for public aquariums of near 1,000g or more (Liveaquaria's 300g recommendation is too small). I cannot comprehend why LFS (even petco) would sell this fish. Most of the specimens sold are only 3-4", small and cute, but this guy will grow into a monster very fast.
You should rehome this guy asap as keeping him in a 40g is severely stunting his growth and increasing his stress level.