I have a 2 tier pond with 2 goldfish and 4 Koi in the lower pond. The top pond overflows and cascades down rocks about 18". Yesterday my wife noticed we have a few baby fish in the upper pond. They appear to be baby Koi. My question is, how did they get up there? We tried keeping a few fish in the upper pond a couple months ago, but they all ended up going over the waterfall into the lower pond. Would it be possible for babies or eggs to survive a trip through a UV sterilizer and through a bio filter? I have a homemade biofilter from a small wastebasket which pours into the upper pond.