From the pictures you posted way back in January/February it looks like he is longer than the tank is wide- that's pretty cruel to have such a huge fish in a small tank like that. I'm not surprised at all that he died.
Why not hold off on buying another RTC until after the pond is built? I'm curious, how do you plan to heat the pond? I saw you live in the UK where I presume it gets pretty cold. Heating that much water is going to be a challenge (read expensive). If you end up not being able to heat it properly, and have to keep them in your small (6') aquarium they will surely go the same way as Arnie. Not trying to be mean, but that is the reality of the situation.
