Our rat breeding foundered on several rocks, so I have a combination of 20-gallon long aquarium and Martin's Cages RT-635 2-story tank topper for sale. The topper is powder-coated black, two levels, with ramps. There are doors into both levels, and the top opens.
http://www.martinscages.com/products/cages/toppers/
Only used for about a month. Tank, cage, and accessories (water bottles, cage coop dish, corner pan) have been thoroughly cleaned with F-10 disinfectant. Minor nibbling to silicone tank sealant in one or two spots. We've modified the upper ramp so there is a "drawbridge" cable to close the ramp from outside the cage. We have the cable and hardware to do the same to the lower ramp, or the cable can be removed with good wire cutters.
$100, OBO. Palatine, IL (NW Cook County), I work in Bartlett, husband works in Vernon Hills. I'll add pictures shortly, currently have a snake out helping me post. :)
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