I've enjoyed keeping "rosy reds" which are just a light yellowish colored mutation of the fathead minnow in my ponds. They are commonly sold (very cheap) as feeder fish in pet stores such as Petsmart. They are very hardy and prolific and have done well living through harsh winters under the ice. Because of their light color they are highly visible. The largest ones usually grow to around 2-1/2". I've kept them with both goldies and koi, no problem. Sometimes in the spring I've had swarms of fry schooling together and highly visible, but as they grow the numbers dwindle. I suppose it has to do with how much food they can find, which keeps them from over running the place.