While asphalt probably contains one of every organic molecule imaginable, all epidemiological data shows that only the fumes of asphalt are toxic. Since you aren't going to be heating your aquarium up anywhere near 400 F (~190 C) this is a non-issue for aquarium purposes.
The glass is not held in by roofing tar -- Type III or IV roofing asphalt softens at close to 200 F (95 C), which would be a real pain to work with and would probably warp the frame. At room temperature roofing asphalt would probably be too brittle to make a reliable seal.
"asphaltum" technically is beach tar -- crude oil which has drifted ashore and has lost an uncontrolled and unspecifiable amount of the volatile fractions. A hundred years ago, before common and more specific terminology was developed, the word was used to refer to any soft semi-refined asphalt.
For a modern material, what you really want is a paving or waterproofing grade asphalt (AC20 or so in US terminology). Ideally, an SBS-modified asphalt to improve the toughness and reduce the processing temperature. Perhaps you can get a sample of such a material from a local waterproofing contractor. There should be a BAKOR representative somewhere around New Hampshire.