Does it have to be tropical? I remember seeing on one of the hugh fearnley-whittingstall programs a fish farm in the UK that was connected to a water cress farm that grow trout without needing to add any extra food. The fish just eat the things that got flushed out of the cress beds. Worked a treat by all amounts. Well worth a google if its suitable
I know of two commercial Koi farms in the UK. One has their open weekend next weekend in Weston-Super-Mare and if you came along you'd get all the info you'd need. Or look at their websites for info - Koi UK and Cuttlebrook Koi.