A friend told me about Landscape Tanks where they combine a water tank and retaining wall into one and it looks pretty neat: http://www.landscapetanks.com.au/
Looks just like a concrete rendered retaining wall... I'm thinking of going with those if it doesn't cost too much
I spoke to these guys at the home show this year, they would make for a perfect aquaponics setup, but they couldn't really advise if the concrete would be fish safer or not as a fish tank... In theory, once cycled should be ok, but still researching that one...
Perfect for thermal retention during winter, and not overheating during summer