The minimum recommended tank size for an adult arowana is no less than 300 gallons. The pond is going to cost anywhere from five hundred-thousands. It all depends on how you build it and where you purchase materials. It is seriously an undertaking to just "build a pond" especially when you live with mom and pop. Space, electricity, filtration, support, etc. the list goes on and on. The heating and filtration are going to cost a boatload alone. If your unable to provide the adequate the space for these fish I suggest you take them back and get some store credit. They will outgrow a 55 gallon in a matter of months. Decisions need to be made quickly