Hi Lellibo,
Hamsters do love to dig, so in any cage that you get it should have a deep layer of substrate for them. There are several different types of substrate such as normal wood shavings or very fine shavings, carefresh, Aubiose, Bed Excel, Cat Litter etc. - but which one you use usually depends on individual preference, and it also depends on what kind of hamster you have. Long Haired hamsters sometimes have problems with shavings and others getting tangled in their fur, especially with males, so some people tend to use something different which is less likely to tangle in the fur - i use Bed Excel for my LH Male Syrians - it is cardboard cut up into small square pieces and it works really well
Most hamsters will also enjoy having a box of Chinchilla Sand in their cages too as they can dig in this and it also keeps their fur nice and clean - but some hamsters will use it as a potty, so you do need to be careful that it doesnt get dirty!
What kind of hamster are you getting? What kind of cage will he or she be living in? How big is the fish tank that you have?
Sorry for so many questions, but you need to get the right cage for the type of hamster you are going to get
How would you attatch the cage to the fish tank? The tubes from the crittertrail / habitrail / rotastak kind of cages are not really suitable for dwarfs with furry feet as they can sometimes have difficulty climbing them. They are not really suitable for syrian hamsters either, especially syrian hamsters who have been bred by breeders who breed to show their hamsters - as these hmsters usually get very big and are usually too big to fit through the tubes!!
Depending on how big the fish tank is, it may make a perfect home for your hamster on it's own, but obviously it would have to be big enough
Sorry for the long post
Tammy x