hmm that is very weird for an aro to sit on the bottom, even when mine is un-active, it just kinda floats around the surface. Mine didn't really eat for the first few days after i got it but it did come arounds and now it will devower anything that touches the surface of the water.
like everyone else has said thr aro sticks enhaced with a bit of garlic, mine also loves the pellets i use for my oscar, frozen and freeze dried blood worms, i dunno just try a bunch of different things, and double chack water params, cuz unless your tank is very heavily planted i don't think you should have no nitrates?????