Juwel Vision 260 - question about litres/gallons

[Icemist]

I was talking to a friend about the Juwel Vision 260 and am struggling to work out the litres that the tank holds. The website states 260 litres / 68 us gallons with the tank measuring 121 cm long x 46 cm wide x 64 cm high.
But when I calculate the actual volume it comes to 356 litres or 94 us gallons.
Why? Is there something I am missing when calculating this?

 

[cynic]

The volume of water is based on a 70 to 80% figure, you will have filters, sand, plants, wood etc that will all take up space in the tank not to mention you wont be filling the tank to the very top.

With the top panels on mine and the bracing the water stops around an inch, inch and a half from the top of the tank. You need this for gas exchange but mainly to allow the volume of your arms. Think about how much time you will be at least elbow deep. I know I'm in mine often pruning plants back for example. It all adds up.

 

[Essjay]

The Vision range is bow fronted so you can't use a calculator intended for rectangular flat fronted tanks. The centre of the front may be 46 cm from the back but the sides will be less than this.

Juwel do seem to call their tanks by their actual size. I had a Rio 125 which held 125 litres when empty. Of course, it held less than that once I'd put in substrate and decor.