I have 4 ponds in the garden Monkey, one, the larger one, contains a Siberian stugeon and a Sterlet (the girl friends idea), in theory these stugeon should only grow according to the size of the environmet in which they in habit, should they sow signs of outgrowing the pond, however, I will simply move them on. Stugeon, a number of varieties, are now readily available in many garden centres and many, like goldfish, have now found their way into british fisheries. A good example of this is Rocky, who lives in the Motorway complex. I don`t agree with the introduction of no indiginous species behing introduced into British waters, because apart from the illegality of it, it does/can create numerous ecological.
Then again many species of problems fish available in British waters are non-endemic