The problem with frogs is that during their mating season, they tend to attach themselves to anything, koi included. I've seen a photograph of a koi where the frog was so tightly clamped to its head that it had dug its feet into the koi's eyes & damaged them.
I've also seen pics of a clamped on frog that was holding the koi's gills shut so that it was suffocating.
Natural ponds are great, just need to keep an eye on your fish.
Posted on Aug 4, 2005, 9:08 AM from IP address 135.196.99.93