I was very interested to read your comments. I have a big golden, about five inches long, and he has managed to dispose of any same species tank mates I put in there. I hadn't been very fond of him, until last night!!
I am in the process of setting up a new tank, four foot long, and it has grown a rust coloured algae on all the surfaces where light falls. I put him in, and in less than twelve hours he has vacuumed all the algae off!! Not sure how he managed in such a short time, but after an hour there were tracks all through where he had been. This morning all the rocks and gravel are back to normal colour. So I am now his biggest fan!! Here's hoping he doesn't attack the other fish when I put them in, and thanks to your warning I won't add any more.
Cheers,
Sam