I'm using part of my garage.
Ceiling got those silver insulation up high against the tiles.
The part I'm using is partition off the rest with shutter board. It does have about 100mm gap on floor level so if I need to sweep or wash out the place it is easy. On top of the rafters are more shutter board making a roof room. All the camping stuff, Christmas tree and things you use once in a blue moon are up there. So the room is rather closed up.
As far as fumes, all paint tins are properly closed and all other chemicals as they should. No problem in that regards. Temperature fluctuations, Not really a problem, humidity is more. I would benefit from the room not being to insulated actually.
The space I used is almost one side of a double garage, So it works out to 5m by 2m wide. The space under the roll up door got steel racks accessible from outside, when you open the garage doors. All the paint stuff, garden, and other what not items you might need once a month is stored there. Directly behind the steel racks is the shutter board wall.
As for space to store all the fish related items you do gather, yes for a sump room. Water changes I do from there. No water spilling inside the house.