Got a dehumidifier today, 30 pint. Glad to say since I have not been in there, mold hasnt moved or gotten worse at ALL. After lots of thinking I have narrowed it down to the floor not being insulated. Concrete pad with 2x4's running across with plywood on top was how it was built, makes perfect sense why mold is growing around the edges only.
My solution, a fun annoying project, would be to remove everything from the shed, remove the plywood floor and 3' up the sides and pour concrete over the existing slab and 2x4's, using the walls as a tank then form the rest and effectively making twin concrete tanks one of each side approx 4x9 for fish and 3x9 for turtle leaving me a middle isle all the way to the back. That would seal and take care of anything for the future. If I went this route for the tank the question would be forms with concrete, or concrete blocks filled in with concrete and rebar....... only consideration is how hard it would be to seal concrete block, alot faster to build with it though.
You can see the mold on the edges near the floor, nothing to crazy yet (my standards) but have no progressed any
Thats not mold on the floor in the first pic, thats dirt and everything from setting everything up when getting everything ready
20150306_163245 by
smetters20, on Flickr
20150306_163259 by
smetters20, on Flickr
Humidity after I opened the door and brought the box in
20150306_163402 by
smetters20, on Flickr
Dehu is set on high @35% I will be out back later tonight to check on it, in the end I will set the fan speed on low/med and may or may not adjust the humidity to 40% just depends on the run time. It has a constant drain setup but I am not going to use it at this time, if I can go a few days without it filling up I will leave it as is, just another excuse to go in the shed at night

It sure as hell is nice to have a breeze in that shed though!!!!
20150306_163709 by
smetters20, on Flickr