You should aim to get as much water in there as possible (in tanks) as it acts as a heatsink. If you have the place loaded heavily with fish the air will become contaminated with waste gasses and will need to be refreshed to make it comfortable to work in. I used to leave the door open for a while to get an air change. It takes very little energy to reheat the air and the water holds most of the heat OK. I never used dehumidifiers but I lined the room out with sheet steel,sealed with RTV at the joints and painted with maf approved mould resistant paint. You could double glaze the windows but if you do it yourself put a bit of the moisture removing chemical between the glass sheets.