On the koi question. As this is my first pond - where I bought the fish to stock - the guy was really after me to do that - because of the algae.
Either the guy was misinformed as to their actually ability to control algae... Or, he was purposefully misleading you to make an easy sale. An all too common practice. Even grass carp, which are much more evolved to feed on aquatic vegetation don't prefer to eat filamentous algae.
So...if algae is not a significant amount of their diet - what is?
Common carp are omnivorous. They can eat and will eat water plants and algae, but prefer to scavenge the bottom for insects and their larvae, crustaceans, benthic worms, mollusks, fish eggs and occasionally small fish. I would guess in most populations, 90%+ of a carp's diet is bottom dwelling organisms. I have watched them feed heavily on mulberries as they fall from shoreline trees and they'll gladly feed on waste grains that fall into the water. There is a river where cows wade into it to drink and also use it as a bathroom. As nasty as it is, the carp are right there picking the partially digested corn right out of the cow patties.
Did I makle an error putting them in there to begin with?
Unless you really like watching them, I'd remove them. A lot of people like having 2 or 3 per acre of pond as they become almost like pets coming to the fish feeder in their ponds. A few usually won't cause a problem, but higher numbers can lead to turbidity issues and they begin to take up large amounts of the available biomass in a pond. Also, if they spawn, most of their young will be brightly colored also and will be easy pickins for the bass in your pond. However, some may be the more natural cryptic coloration. These young koi may survive and over successive generations look more and more like carp, causing issues.
