10-13-2008, 07:55 PM
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Koi are omnivorous. They may eat your guppies, but the guppies just reproduce so fast you don't notice that the koi eat some of them. The tetras, on the other hand, would not reproduce very easily (the koi would definitely eat their eggs) so their numbers would probably dwindle over time.
What temperature is the water? Both koi (especially your acclimatized tropical koi) and guppies will do well at temperatures around 74*F, but neons would suffer at this temperature. They need 78-80 to thrive.