if ur 50 is 18" it will give u more time. what length? everntually ull want a 2ft wide tank. 6 months give or take hopefullly with ur 50g but it depends on length and width(wich u state is 18")
A 55 breeder tank would be ok as a step up for the fish (45 breeder tank also would work). After this look into a 120 gallon or larger. At 120 you could potentially keep the fish for a very long time. Really depends on many factors but in general this seems to work. As a rule think wide and shallow. Wide shallow and long is a triple bonus worth looking for and investing in..