You should be thinking about upgrading now! Honestly nothing can prepare you for how fast they grow, I got my 1st 1 at 2" and it was a foot long in 3 months! And I agree about the footprint but i don't see how a pond or tank are any different if they are the same footprint? I am building a 2550 litre out of plywood that will be ready in a month or 2 and I still think I will need to upgrade in the future, the RTC is truly a monster fish.
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Even tho this post is old i will respond.
A tank that is 7x3x3 is roughly the same gallons of a pond/tank that is 6x5x2. however the 6x5x2 is much more suitable then the 7x3x3
The difference between this pond and a tank that hold the same amount of water is the footprint, this pond has twice the amount of floor space to swim around, and these are very strong walled ponds. Next pond will be a lot bigger.