My mate who kept marine fish in his "shed" has this issue with high humidity, in winter condensation & mould appearing on walls despite the winter heating and despite the tanks being covered.
Solved by combination of large extractor fans (12 inch ?) in the roof and two part air conditioning units. You need the two part units as it will extract moisture from the air inside and pump the condensed water outside. He eventually fitted a drain pipe as originally the condensate dripped onto a flower bed, but only large quantities of slime and moss grew.
Well i wanted to avoid puting vents of any kind into the room. I am heating the room rather than the tanks which is saving a lot on electricity and also much safer as its less likely to have one fail and boil a tank (its happened to me before!). So if i put in vents, i will be venting off all the hot air!
THere must be a source of the humidity....water cannot just appear! haha. I am wondering if its moisture still in the walls from when the humidity was high. But sounds like i'm not the first to find this!
The dehumidifier works fine, but the issue is the cost of running it! It basically doubles the running cost of the fish room when its on.
I need to find a solution