I insulated the walls with Batts just like you did. I then lined the whole room with 50mm Polystyrene completely wrapped with Polyethylene plastic sheet, and once installed EVERY gap/join was tapped to seal things up. My fish room still has the dry dust on the roof and walls from when I built it - 15 years ago!!! It has never had moisture problems. To heat it I went to an electrical store and purchased one of those wall mounted thermostats and wired it into a female plug, then I plugged in a 1200W fan heater, set the thermostat to your desired temp and bingo... one heated fish room. It is very efficient and cheap to heat, the key to it all - stacks of insulation and 100% sealing ( E.G: no draughts = no heat out & no cool outside air in ).
If you have any doubts then ask anyone on here who has been to my room and seen the results :dnc1: