[Crystalroborovski]

Not too sure how to connect a tank to a bar cage.

As for the substrate: how big is the smaller cage? I wouldn't have the whole thing with sand as substrate just because it doesn't hold burrows at all so your hamster wouldn't be able to make burrows in the smaller cage. I would opt for paper based substrate rather than aspen because some hamsters can be allergic to or develop an allergy to aspen bedding. Plus, paper based substrates are a lot softer.