well, i say turn off the lights b/c my Jar used get nuts when the lights were on...now at 10"+ he still sometimes gets crazy about the lights...
i just recommended that to you b/c your aro is at such a young age and he doesnt need any more stress than he's already receiving..(getting adusted to the new tank/tankmates/you/etc.)
even if he doesnt seem stressed..why chance it? ...i wouldnt..