sam, how many tanks do you know of that are in kitchens bathrooms or laundry rooms? His circuit may very well be 20A but that still does not change the fact that most homes are wired with 15A recepticle branch circuits.
Furthermore the 20 AMP laundry recepticle is only to be used for laundry equipment and can include only the wiring device needed for that equipment, and no other outlets.
The bathroom 20A branch circuit is required and CAN NOT supply any other load or outlet outside of the bathroom (a light in the bathroom is permited).
The 20A circuits required in a kitchen or dining area are listed as "small appliance" outlets and can have no other outlets outside of these areas.
So yes, of course there are parts of the home that do have 20A circuits, however those circuits are only in those rooms and are not suitable for a load such as an aquarium.
Furthermore, in an older home, the 20A branch circuits listed above were NOT a requirement.
Of course some localities may have codes that go beyond what is required by the NEC in terms of equipment and inpsection.... this is also a rare thing nd usualy the result of a local politician and contractor being related or in business together

Thats for another thread....
Walk into a $700,000 home and tell me how many 20A circuits you find? If the buyer did not specify an upgrade, they will not be there.
Bean