Soft tooth brush.. 🙂 George farmer has a very nice maintenance video roaming around. Where you see him do an all over tank clean session.. A little search and you'll find it if intrested. Also those little cheap brushes used to apply glue, these have a bit tougher/harder hairs than paint brush, are also very good to use.. If you take a flat brush you can gently bend it in an angle to make difficult corners easier to reach. 🙂
Oh ps, i read the tank is empty, chlorine will remove algae very good, but still need a soft brush to get in between glass and kit if there is any algae between.