Afraid I'm going to have to disagree a little there A fully grown goldie wouldn't be able to get up much of a head of steam in that size, it's only 1m diameter. Admittedly it could go round in circles Considering the size that Howie has managed to get to I'm struggling to imagine 5 or 6 of him in 450 litres. It's less volume than my Rena tank.
I've just measured out 1m on my desk, sorry but that's small!
It would be a good starting point for maybe 3 or 4 goldies while you were building a bigger pond but as a long term home I'm not convinced
Wouldn't it be better with some sticklebacks or something if it were outside?
Inside now ... as a temperate or tropical pond I'm sure there are many more options