Lol - yes a tank is a bit big to get outside
As long as you've cleaned it well with pet disinfectant that's the main thing but I'd suggest rinsing that off well as well after it's done it's job for a couple of hours - removing any scent or residue. And then just let it air for three weeks. Should be fine then.
I'm not sure there is a lot of difference between having windows open and putting something outside in the garden
Depending on whether you have a garden or whether it's in a smokey built up area or something! Shouldn't get any bugs and if it did get a bug in you just wipe it.