The display tank is 78x32x34 which is about 370 gallons plus the two sumps total about 140 gallons , plus the other chambers holding more water, skimmer, calc reactor, etc. makes up for just over 500 gallons.
All make up water is kalkwasser RO/DI water. Add iodine and strontium to keep levels constant.
Not sure why photos are upside down. I think when you click on them the display properly . Sorry .