Identifying a fish

Hi everyone,

I have a fish in my tank at home which has been growing at quite a fast rate
about 10mm per month
I suspect the fish is a female Koi Carp. It was originally purchased as a
feeder fish, my yabby (freshwater crayfish) wasnt interested in eating the
fish instead the fish eats anything in sight and continually sifts through
the gravel in the tank in seach of food. it also grazes on water plants
including broad leaf plants.

The fish looks like the ones pictured (esp the silver one) at this link I
found http://www.harbinson.fslife.co.uk/koi2.jpg

Question how big can these fish get, or how likely is it to continue growing
like this? Currently its about 12cm long.

Im at the point where I need to decide to get a bigger tank / or maybe a
pond (cant really afford new tank & filter etc) or find a new home for the
fish.

thanks scott