Not trying to discourage you as all new aquarium enterprises are good for the industry but I think you will have some challenges ahead. With the improvements in glass and acrylic manufacture, adhesives and international transportation, bigger tanks are not as difficult to obtain as they used to be. A 12 foot long x 4 foot wide x 2 foot high all glass tank can be locally purchased under $2K and isn't really considered a large tank for the demographic you seek.
Sheet metal panel bolt together water tanks are available in 1 meter square increments as long, wide or tall as needed. Already meeting Australian structural standards. For an aquarium, industrial grade epoxy coatings with the steel framing going on the outside.
For in between or specific custom dimensions, HDPE sheets screwed to a steel frame and plastic welded seams are getting popular. All screws are covered over in HDPE.
AND mainland China is producing fantastic glass and acrylic tanks. A fully made tank is only limited by container size (12m). Their labour and factory costs can't be beaten. And even cheaper if ordered by sheets. I brought in 16 foot x 4 foot x 4 foot 19mm toughened glass SGP laminate (40mm) glass sheets which were so cheap, the freight was the most expensive component. And I'm talking about the freight from local port to my house, not the international shipping cost.
To get around your third party warranty issue, you could a) sell without warranty, b) personally fly out and install each (which I'd recommend if you want to maintain quality control and word of mouth service as a start up company or c) use a trained up reseller in each capital city.
Also, rather than installing chillers or UV or solar light tubes, look into chemically removed nitrates so water changes are significantly reduced /eliminated. Contact Mike Hanrahan of AustMarine seafood tanks. There may be more recent technology developments.
Good luck.