Your new Dat might fight with Oscar (but not killing each other, I hope) if they're same size. But if your new dat is stable, I bet you'll be liking your dat over oscar. I have the same size oscar and dat in my tank, I saw them chasing each others once in a while but my oscar is just for show off only, he's not really that tough but the Dat is. I also have 10 years old huge Koi too but eat too much at anything drop in the tank. So I get rid of it. So I'll say Dat will be good tank. Go for it.