If you open a tetracycline capsule into a small container of water (about 1/2 gallon) and soak the infected turtles in it over night, you will find that the fungus almost disappears just by doing that. Then lowering the PH and continuing with the antibiotics for a few more days should knock it out.
Oh yeah, I know how frustrating it is to try and raise a 'show specimen' only to have it lose a toe or two to fungus!