There are certainly scads of dowels in the two tiers under the fishbowl.
This cake would be driven to the party site in two sections.
Then the fishbowl only needs a solid acrylic plate under it and over it to be stable. That's one reason for the generous piped borders on it.
The cake was made by Amanda Groom, in the UK. I believe this is she:
That doesn't show the cake, but you could e-mail her and ask her how she did it.