ok so i recently picked up a new arowana on thursday because my old one died. hes been doing great in the tank until yesterday i noticed him not swimming. hes still alive. but he seems to just hang out in the water current near the filter. and every now and then ill catch him rubbing his back on parts of the filter. like he'll let the current take him and his back will glide against the filter part then he'll swim forward a little and do it again. also he hasn't really been eating much at all. is it possible for fish to get an itch and hes just scratching his back? or is he infected with something?