Several months ago I had an 800 gallon pond installed. I live in Hawaii, and the pond is tropical, lava surrounded with a small waterfall. I started with 8 small feeder-type goldfish and one large koi I was given. I also had a pot with waterlilly in it and this pot had several of what are commonly called (here, at least) mosquito fish. They are very small (about an inch long), black, and used to control mosquitos. So, I put a few of them in the pond also. Imagine my surprise when a few months later several very large black fish appeared! They had been invisible, as the pond bottom is black stone, and surrounded by black lava walls. It seem the small mosquito fish have grow large - as in several inches long, and there are several of them. The biggest are bigger than my goldfish, which have become quite big. So, my question is - what in the heck are these big black fish???? A friend said I should just throw them out, but I can't stand the thought of killing anything. Another weird thing - one of the black fish slowly turned orange, and now has only a small amount of black on him. I know each of my goldfish (they have names) so there is no mistaking that these black fish were the little ones I put in. Also, the goldfish have reproduced twice already, and so I have 8 more smaller goldfish. I would appreciate knowing what these black fish are, and if they will continue to grow. They add to my fish load and I don't want to get another koi (although I really want to) if it would be too much. Mahalo (Thanks!)